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Commons license.
 

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PROFESSOR: All right.
 

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It's 12:05, so why don't you go
ahead and take 10 more seconds

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on the clicker question today.
 

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This is about the
periodic trends that we

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discussed on Wednesday.
 

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So specifically, what we're
asking here is as we go across

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the periodic table, we want
to consider which has the

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smaller ionization energy.
 

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All right.
 

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So, let's focus our attention
up here now, whether it's

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between aluminum or whether
it's between phosphorous.

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And I also wanted you to
identify why it's also

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important to understand why we
have these trends, not just to

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memorize the trend itself.
 

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So, it turns out that the
majority of you got the

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correct answer, which
is that it's aluminum.

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The reason it's aluminum is
because aluminum has a lower z

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effective, so it's not being
pulled in as tightly by the

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nucleus, and if it's not being
pulled in as tightly, you're

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going to have to put in less
energy in order to ionize it,

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so that's why it's actually
going to have the smaller

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ionization energy.
 

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So it looks like not too many
more than half of you got this

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correct, so make sure you can
look at your periodic table and

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figure out how to think about
ionization energy in terms of z

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effective, not just in terms of
memorizing what that trend is.

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All right, so we can go to
today's notes, and in terms of

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the notes, what we're going
to start with is finishing

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material that's going to be
relevant for exam 1, and I told

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you on Wednesday that actually
I'd give you some information

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today in terms of what you need
to do to prepare for exam 1.

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So you should have gotten two
handouts as you came in, and if

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you didn't, please raise your
hand and a TA will come to you

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and get you that
second handout.

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But the second one says "exam
instructions and logistics."

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So, if everyone can
pull that out.

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So this is going to tell you
pretty much everything you need

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to know in terms of getting
ready for the exam, which

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is next Wednesday.
 

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So when you go home today or
some time this weekend, make

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sure you read this
page in detail.

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I'm just going to go over a
few of the main points here.

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So, the first is that what
we're going over notes today

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and also on Monday, the exam
material ends at the end

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lecture notes from lecture 9.
 

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So that was Wednesday's
class -- not at the end of

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Wednesday's class, but at the
end of the lecture notes.

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So we're going to finish
up with those today.

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I'll be really, really clear
when we get through them, and

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that's where you can stop in
terms of studying for this.

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Also, on everything that was
on problem-sets 1 through 3.

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So, you turned in p-set 3
today, but we'll have the

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answers posted for you this
afternoon, so you can start

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studying from p-set 3, even as
early as tonight, if you want

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to, because those answers
will be there for you.

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So, in terms of what it is that
you need to prepare and bring

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with you for the exam, you need
to bring your MIT ID,

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especially if you haven't been
showing up regularly to your

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recitations and you aren't 100%
sure if your TA knows exactly

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who you are, you need to make
sure you have your

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MIT ID with you.
 

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You can't take the exam unless
you're registered for the

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class, and we need to make
sure we can verify that.

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You also need to bring a
calculator, of course, because

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we'll be solving problems
that involve calculations.

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You can bring any calculator
you want, we don't actually

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have restrictions for
calculator types here, but what

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you can't do, is you can't
program any relevant chemical

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or information about
constants in there.

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It's OK to use certain
fundamental constants that come

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in a lot of calculators, so
there's nothing we

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can do about that.
 

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That's OK.
 

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If you're wondering what's OK
or what's not OK, it's very

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clearly written out in this
handout, so make sure you read

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through it, because it is your
responsibility to make sure

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that your calculator does not
have anything extra

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programmed in it.
 

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And if you have your calculator
all set up as you love and you

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don't want to change it, then
maybe you should just go and

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get an $8.00 scientific
calculator that doesn't have

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any of the graphing functions,
because you don't actually need

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them, so that's a better option
for you, you can

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do that as well.
 

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And I had mentioned several
times that you do not need to

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memorize the majority of the
equations and you don't

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need to memorize any
physical constants.

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So if you flip the info page
over on the back here, what

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you'll see is the periodic
table, this is the same one

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that I've handed out in the
last two lectures -- the

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periodic table without any
electron configurations.

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This is exactly the sheet
here, it's exactly what

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you'll get on exam day.
 

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You'll also see that they have
all the physical constants that

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you're going to need, and also
a bunch of the actual equations

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that we've been using in the
first couple weeks here.

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So you don't need to memorize
any of this, you're actually

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going to be handed this.
 

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There are a few equations that
you need to memorize -- those

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are the very simple -- very,
very simple equations, such as

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e equals h times nu --
hopefully you don't have to sit

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down and try to memorize
that, hopefully we all

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know that already.
 

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But just to be really clear,
I've written out exactly which

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equations you do have to
memorize on the front here, so

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long as you know those, the
rest you can just look up.

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And in terms of using these
equations in solving problems

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on the exam, and also using
these constants, make sure if

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you think there might be any
chance you're going to get any

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little part of a problem wrong
or do a calculation

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inaccurately, you need to write
out every single step of your

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thinking as you write
out these problems.

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We can't give you any partial
credit whatsoever if we can't

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see your thought process.
 

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So it's important to write out
the equation you use, you need

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to write out the constants
that you use to fill

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in that equation.
 

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And that we need to see your
work to get full credit, and

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then especially if you get
things wrong, we need to know

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where it went wrong, because we
do try to give as much partial

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credit as possible in these
exams, since there are a lot of

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places where small mistakes can
result in the wrong answer.

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So also, along those lines in
terms of test taking, make sure

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you also box your answers and
that you keep track of

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significant figures and that
you also remember to

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include your units.
 

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These are just little things
that can add up, so you just

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want to make sure you're
on top of those.

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And your TAs on Tuesday are
going to share a lot of other

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types of sort of exam
strategies in thinking about

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how you can approach an exam
when we're in a time situation

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like we are, so they'll share
some of their experience with

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you in terms of taking
these timed exams.

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So, in terms of practicing this
weekend, I mentioned that

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instead of getting a
problem-set today, what I am

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going to be posting is
optional extra problems.

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So they're optional, but
they're very, very, very highly

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encouraged that you do these,
because this is going to give

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you practice for the types of
problems that are going

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to be on the exam.
 

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We're also posting a practice
exam for you to take, so after

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you're completely done your
studying, it's good to have

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everything done before you take
the practice exam, and then sit

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down just with this sheet here
and your calculator, and

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ideally a timer, and make sure
you can do the practice exam in

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the allotted amount of time.
 

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So that way you can have an
idea if, oh, I do really

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understand this but I'm a
little bit slow, maybe I need

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to practice this one type of
problem a little bit longer so

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I can get up to speed so I'm
going to be able to get through

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all this in terms
of the exam time.

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All right.
 

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So let's move on to
today's topics.

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So, as I said, we're finishing
up with what we left off with

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yesterday -- or excuse
me, on Wednesday.

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This includes atomic
radius and the idea of

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isoelectronic atoms.
 

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So that's going to be the end
of the exam 1 material, and

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then we'll move on to exam 2
material, which is kind of

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exciting, because we've been
talking about just individual

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atoms and ions up to this
point, and now we can talk

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about molecules, so we're going
to start talking about bonding.

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So for some you that are less
interested in maybe the

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physical structure of an
individual atom, now some more

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exciting material for you might
be coming up if you like to

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think about how, instead,
molecules behave, either within

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bonding, within themselves, or
with other molecules, that's

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what we're going to be heading
to in this next unit.

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So, we need to finish up
with periodic trends.

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And first, on your lecture
notes, I start with

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atomic radius.
 

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I was so proud of myself
getting the lecture notes

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finished early and handing them
in to CopyTech, and then

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I realized we didn't do
electronegativity on Wednesday.

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So, if you can flip your
lecture notes over and just

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write on the blank space, we're
going to cover

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electronegativity first here,
and specifically, you can go

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back and fill this in to your
lecture 9 notes, if you want to

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stay organized, but I just
suggest just writing it on

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lecture 10 notes now
and going back.

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You can still keep organized,
which hopefully most of you

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like to do, and get it in the
right place in the notes.

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So, when we're talking about
the idea of electronegativity,

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essentially what we're talking
about is the ability for an

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atom to attract electron
density from another atom.

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So it's just a measure of how
much does one given atom want

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to pull away electron
density from, let's

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say, an adjacent atom.
 

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So it's actually very related
to what we're talking about

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when we said electron affinity,
and it's also related to

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ionization energy, and we can
call electronegativity by

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symbol here, and it turns out
that it's going to be

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proportional to 1/2 of the
electron affinity of a

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given atom, plus the
ionization energy.

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So, in other words, we can just
think of electronegativity as

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being the average of that
ionization energy and

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the electron affinity.
 

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This should make sense, because
if an atom has a very high

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electron affinity, that means
it's really happy taking an

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electron from another atom,
or taking a free electron --

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that that's very favorable.
 

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If something has a high
ionization energy, it means

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that it really, really,
really does not want to

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give up an electron.
 

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So you can think about how
these 2 things combined are

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going to be electronegativity,
which is a measure of how much

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an atom wants to pull electron
density away from another atom.

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So, if we think about
electronegativity as a periodic

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trend, we can just draw our
nice periodic table here, and

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let's separate it
into quadrants.

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So if we think about the upper
right hand part of the

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quadrant, well, this is where
we're going to have high

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electron affinity and high
ionization energy, so we're

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also going to see high
electronegativity here.

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And in contrast, in the lower
left hand part of the periodic

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table, these 2 quantities are
low, so also what we're

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going to see is low
electronegativity.

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And if we talk about what's
going on in areas, or with

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atoms that have high
electronegativity, and we think

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about whether they're electron
donors or electron acceptors,

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what would you expect for an
atom that has high

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electronegativity?
 

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Is it going to be an
electron donor or acceptor?

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Great.
 

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Yup, it's going to be an
electron acceptor, it wants to

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accept electrons, it wants to
accept electron density.

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So, in contrast, if it has a
low electronegativity, this

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then is going to be
an electron donor.

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All right, so it's very common
to talk about electronegativity

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of different atoms, and you
can look up tables of these.

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Often what you'll see is not a
table based on this definition,

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but something that's called the
Pauling definition of

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electronegativity, but it's
exactly the same idea and the

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same trend as this more
numerical way to think about

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what the meaning of
electronegativity is.

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All right, so now we can move
on to the start of today's

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notes, which is atomic radius.
 

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So this, in fact, is going to
be our last principle that

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we're going to talk about in
terms of periodic trends.

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So this is actually the most
straightforward, so sometimes

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it's nice to end with the
easiest concept, and that's

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what we're doing here.
 

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And if we're talking about
atomic radius, essentially

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we're talking about
atomic size.

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And immediately it should
probably come into your head

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that we don't actually have
an atomic radius that we

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can talk about, right?
 

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What I just spent many lectures
discussing is the fact that we

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can not know how far away an
electron is from the nucleus,

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so we can't actually know the
radius of a certain atom.

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And that's true.
 

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An atom's not a defined
sphere, for example.

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We can't define it as an
exact radius in terms of

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the definition we might
think of classically.

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So, keep that in mind when
we're talking about atomic

265
00:12:36 --> 00:12:39
radius, I'm not suddenly
changing my story and

266
00:12:39 --> 00:12:41
saying, yes, we do have
a distinct radius.

267
00:12:41 --> 00:12:43
Instead, what people have done
is come up with different ways

268
00:12:43 --> 00:12:46
to think about how they
can define a radius.

269
00:12:46 --> 00:12:48
And one common way to think
about it, is to think about the

270
00:12:48 --> 00:12:52
value of r, or the radius,
below which 90% of that

271
00:12:52 --> 00:12:55
electron density is
going to be contained.

272
00:12:55 --> 00:12:57
So we're not saying
it's all the electron

273
00:12:57 --> 00:12:58
density, it's just 90%.
 

274
00:12:58 --> 00:13:01
Because we know as we go to
infinity, even though the

275
00:13:01 --> 00:13:04
density gets smaller and
smaller and smaller, we still

276
00:13:04 --> 00:13:07
have electron density very
far away from the nucleus.

277
00:13:07 --> 00:13:11
So, what we're going to define
is just let's just capture 90%

278
00:13:11 --> 00:13:12
of that electron density.
 

279
00:13:12 --> 00:13:15
So, that's one way to think
about it, and there's also

280
00:13:15 --> 00:13:17
another way, and this is the
way that your book presents it.

281
00:13:17 --> 00:13:21
If you, in fact, have two of
the same atom right next to

282
00:13:21 --> 00:13:23
each other, let's say you have
a crystal, or let's say you're

283
00:13:23 --> 00:13:27
talking about a metal, what you
can do is just look at the

284
00:13:27 --> 00:13:31
distance between the two
nuclei, and split that in 1/2,

285
00:13:31 --> 00:13:33
and take the atomic
radius that way.

286
00:13:33 --> 00:13:35
So, these are two different
definitions of how to think

287
00:13:35 --> 00:13:38
about atomic radius, but really
what you find when these are

288
00:13:38 --> 00:13:42
measured is they come up with
almost the identical values, so

289
00:13:42 --> 00:13:45
there are tables you can look
up of atomic radii and see

290
00:13:45 --> 00:13:48
these values, and you can trust
them that, yes, they work for

291
00:13:48 --> 00:13:51
both this definition and for
this definition here,

292
00:13:51 --> 00:13:54
in most cases.
 

293
00:13:54 --> 00:13:56
And what we've been talking
about with all of these

294
00:13:56 --> 00:14:00
properties are, of course, how
can we figure out what that is

295
00:14:00 --> 00:14:02
for a certain atom by looking
at the periodic table, so we

296
00:14:02 --> 00:14:04
want to think about the
periodic trend for

297
00:14:04 --> 00:14:06
atomic radius.
 

298
00:14:06 --> 00:14:09
And we know as we go across a
row in the periodic table,

299
00:14:09 --> 00:14:12
what's happening is that z
effective or the effective pull

300
00:14:12 --> 00:14:14
on the nucleus is increasing.
 

301
00:14:14 --> 00:14:18
So would you expect, therefore,
as we go across a row for

302
00:14:18 --> 00:14:23
the atomic radius, to
increase or to decrease?

303
00:14:23 --> 00:14:23
Good.
 

304
00:14:23 --> 00:14:24
OK, yes.
 

305
00:14:24 --> 00:14:27
We are expecting to see that it
decreases because it's feeling

306
00:14:27 --> 00:14:30
a stronger pull, all the
electrons are being pulled in

307
00:14:30 --> 00:14:33
closer to the nucleus, so that
atomic size is going

308
00:14:33 --> 00:14:35
to get smaller.
 

309
00:14:35 --> 00:14:37
This is in contrast to
what's happening as we go

310
00:14:37 --> 00:14:39
down a periodic table.
 

311
00:14:39 --> 00:14:42
So as we go down we're now
adding electrons to further and

312
00:14:42 --> 00:14:45
further away shells, so what
we're going to see is that the

313
00:14:45 --> 00:14:47
atomic radius is going to
increase as we're going

314
00:14:47 --> 00:14:52
down the periodic table.
 

315
00:14:52 --> 00:14:54
And we can look at
an example here.

316
00:14:54 --> 00:14:57
If we start in the upper left
hand corner of the periodic

317
00:14:57 --> 00:15:00
table with lithium, you can see
that as we go down the table,

318
00:15:00 --> 00:15:03
what you're seeing is that that
atomic radius is actually

319
00:15:03 --> 00:15:05
increasing, as we would expect.
 

320
00:15:05 --> 00:15:09
Whereas, if we go across a
row, what we see is that the

321
00:15:09 --> 00:15:11
atomic radius is decreasing.
 

322
00:15:11 --> 00:15:14
So, again, this is one of the
more straightforward trends.

323
00:15:14 --> 00:15:15
You just need to remember
what's happening to z

324
00:15:15 --> 00:15:18
effective, which really tells
us what's happening with all

325
00:15:18 --> 00:15:21
the trends, and once you know z
effective, you can figure out,

326
00:15:21 --> 00:15:24
for example, what direction the
atomic radius should

327
00:15:24 --> 00:15:26
be going into.
 

328
00:15:26 --> 00:15:28
So, that's it for
periodic trends.

329
00:15:28 --> 00:15:29
We have talked about
four different ones.

330
00:15:29 --> 00:15:33
We talked about ionization
energy, electron affinity, we

331
00:15:33 --> 00:15:36
talked about electronegativity,
which is just kind of a

332
00:15:36 --> 00:15:39
combination of the first two,
and then ended with

333
00:15:39 --> 00:15:41
atomic radius here.
 

334
00:15:41 --> 00:15:44
And what you might have noted
is although we described how to

335
00:15:44 --> 00:15:47
make predictions about these
properties, I didn't talk too

336
00:15:47 --> 00:15:49
much about what it actually
means, what the ramifications

337
00:15:49 --> 00:15:51
of these different
properties are.

338
00:15:51 --> 00:15:53
And the reason we didn't do
that is because we're actually

339
00:15:53 --> 00:15:57
going to spend much of the rest
of the course relating these

340
00:15:57 --> 00:16:00
different properties to the
properties of molecules in

341
00:16:00 --> 00:16:03
terms of bonding, and also in
terms of chemical reactions.

342
00:16:03 --> 00:16:07
So, for example, if we have a
very electronegative atom

343
00:16:07 --> 00:16:11
within a certain molecule, what
you'll actually find is that it

344
00:16:11 --> 00:16:16
does affect how the molecule is
going to take part in different

345
00:16:16 --> 00:16:17
chemical or biological
reactions.

346
00:16:17 --> 00:16:20
And this will become more and
more clear as we actually

347
00:16:20 --> 00:16:22
talk about these reactions
and talk about bonding.

348
00:16:22 --> 00:16:26
But you need to be able to
predict what kind of properties

349
00:16:26 --> 00:16:28
a certain atom's going to have
within a molecule, whether

350
00:16:28 --> 00:16:30
you're talking about something,
for example, that's very

351
00:16:30 --> 00:16:32
electronegative, or something
that is not electronegative at

352
00:16:32 --> 00:16:35
all, it is going to make a
difference in terms of thinking

353
00:16:35 --> 00:16:37
about how molecules are
structured and also how they

354
00:16:37 --> 00:16:41
interact with other molecules.
 

355
00:16:41 --> 00:16:43
However, I can give you at
least one example while

356
00:16:43 --> 00:16:46
we're still on just
talking about atoms.

357
00:16:46 --> 00:16:48
So we haven't gotten to
molecules yet, we're just

358
00:16:48 --> 00:16:52
talking about single atoms or
single ions, but what's nice is

359
00:16:52 --> 00:16:54
just talking about this very
straightforward principle

360
00:16:54 --> 00:16:56
of atomic radius.
 

361
00:16:56 --> 00:17:00
We can already use that in
terms of single ions to think

362
00:17:00 --> 00:17:03
about a really complex
biological issue, which is to

363
00:17:03 --> 00:17:05
talk about ion channels.
 

364
00:17:05 --> 00:17:07
So, that is just a quick
example for some of you, you

365
00:17:07 --> 00:17:10
might be very familiar with ion
channels, others might not know

366
00:17:10 --> 00:17:13
what these are, so I'll just
tell you quite briefly that ion

367
00:17:13 --> 00:17:17
channels are these very massive
transmembrane proteins.

368
00:17:17 --> 00:17:21
Essentially, what they are is
it's a protein that spans

369
00:17:21 --> 00:17:23
the membrane of a cell.
 

370
00:17:23 --> 00:17:26
And what they do is they
regulate the influx of

371
00:17:26 --> 00:17:27
ions across that cell.
 

372
00:17:27 --> 00:17:30
So the influx of ions from the
outside of the cell to the

373
00:17:30 --> 00:17:32
inside of the cell,
for example.

374
00:17:32 --> 00:17:35
And you can think of ion
channels as being gated, by

375
00:17:35 --> 00:17:39
gated it means the gate can be
closed and no ions are going

376
00:17:39 --> 00:17:41
through, as in this case here.
 

377
00:17:41 --> 00:17:45
Or you can talk about the gate
being open, and in this case,

378
00:17:45 --> 00:17:48
you can see that you will
have an influx of ions.

379
00:17:48 --> 00:17:52
So, ion channels are important
for maintaining a voltage

380
00:17:52 --> 00:17:54
difference between the inside
of the cell and outside of the

381
00:17:54 --> 00:17:57
cell, and they're found in all
sorts of cell types in your

382
00:17:57 --> 00:18:00
body, but if you think about
where they're most prevalent,

383
00:18:00 --> 00:18:04
it turns out that they're most
prevalent in muscle cells and

384
00:18:04 --> 00:18:07
also in nerve cells,
so in your neurons.

385
00:18:07 --> 00:18:10
And essentially, what they do
in neurons is they underlie

386
00:18:10 --> 00:18:14
those nerve impulses, or those,
essentially what we call

387
00:18:14 --> 00:18:18
electrical signaling between
neurons -- you might also call

388
00:18:18 --> 00:18:21
that the action potential
of the neurons.

389
00:18:21 --> 00:18:24
So essentially, they regulate
this action potential, and they

390
00:18:24 --> 00:18:28
do so by helping to establish
and then control the voltage

391
00:18:28 --> 00:18:31
gradient within the cells.
so, essentially, they're

392
00:18:31 --> 00:18:35
establishing or controlling or
changing the difference between

393
00:18:35 --> 00:18:39
the charge inside the cell
and the charge outside cell.

394
00:18:39 --> 00:18:42
And when we talk about any type
of ion channel, there are just

395
00:18:42 --> 00:18:44
tons of different kinds of ion
channels, and you can

396
00:18:44 --> 00:18:46
characterize them in a
few different ways.

397
00:18:46 --> 00:18:48
So, for example, you could
characterize them in terms

398
00:18:48 --> 00:18:51
of how they're gated, and
basically how they open or

399
00:18:51 --> 00:18:54
close -- that's one way to
talk about different types.

400
00:18:54 --> 00:18:56
Another way to talk about
different types is to

401
00:18:56 --> 00:18:58
think about which ion
they're selected for.

402
00:18:58 --> 00:19:02
And all ion channels are
selective for a single type of

403
00:19:02 --> 00:19:06
ion, and we can think about how
that selectivity takes place,

404
00:19:06 --> 00:19:08
and that's where this idea of
atomic radius is going to

405
00:19:08 --> 00:19:10
become very important.
 

406
00:19:10 --> 00:19:13
So, for example, if we look at
sodium channels, and sodium

407
00:19:13 --> 00:19:16
channels are some of the
particularly prevalent ones

408
00:19:16 --> 00:19:19
when we're talking about
neurons, if you think about the

409
00:19:19 --> 00:19:23
cell membrane, and this little
green cartoon is me trying to

410
00:19:23 --> 00:19:27
show a sodium channel here, and
in this case, you can see that

411
00:19:27 --> 00:19:30
it's closed, such that no
ions are getting through.

412
00:19:30 --> 00:19:33
However, when that gate is
opened, the sodium channel is

413
00:19:33 --> 00:19:36
now going to be incredibly
selective and only let through

414
00:19:36 --> 00:19:39
sodium ions and no
other type of ion.

415
00:19:39 --> 00:19:41
And this is really interesting
to think about because you can

416
00:19:41 --> 00:19:44
imagine in our body we have
concentrations of all types of

417
00:19:44 --> 00:19:47
ions, and specifically, some
seem very, very similar

418
00:19:47 --> 00:19:48
to each other.
 

419
00:19:48 --> 00:19:50
So we could think about
comparing the potassium

420
00:19:50 --> 00:19:52
ion to a sodium ion.
 

421
00:19:52 --> 00:19:54
They have the same
charge of plus one.

422
00:19:54 --> 00:19:57
The only thing that's different
is that they're one down on the

423
00:19:57 --> 00:20:02
periodic table, potassium is
down one row, so it's going to

424
00:20:02 --> 00:20:05
be a little bigger, but when
we're thinking about size, it

425
00:20:05 --> 00:20:07
maybe does not seem that
significant to talk

426
00:20:07 --> 00:20:08
about the size.
 

427
00:20:08 --> 00:20:10
But what we find
out is that it is.

428
00:20:10 --> 00:20:13
So, what happens, this is
another view of a sodium

429
00:20:13 --> 00:20:15
channel, so this is actually
looking a little bit more

430
00:20:15 --> 00:20:17
at the protein structure.
 

431
00:20:17 --> 00:20:20
What all of these channels have
is what's called a selectivity

432
00:20:20 --> 00:20:24
filter, so this filter filters
out the type of ion that's

433
00:20:24 --> 00:20:25
going to be allowed through.
 

434
00:20:25 --> 00:20:27
And there's two parts
of the filter.

435
00:20:27 --> 00:20:30
First we need to select
for actual charge.

436
00:20:30 --> 00:20:33
So the way that it does this is
the filter is actually lined

437
00:20:33 --> 00:20:35
with all of this negative
charge, and for those of you

438
00:20:35 --> 00:20:39
that are more into biology or
biochemistry, that's because of

439
00:20:39 --> 00:20:43
negatively charged amino acid
residues, but all you need to

440
00:20:43 --> 00:20:45
think about is that it has
negative charge in the inside

441
00:20:45 --> 00:20:49
of this pore, and what happens
then is that if something has a

442
00:20:49 --> 00:20:52
positive charge, it's going to
be stabilized to enter this

443
00:20:52 --> 00:20:55
pore, whereas any negative ions
are going to be repelled.

444
00:20:55 --> 00:20:58
So that's the first step in
being selective, but now how do

445
00:20:58 --> 00:21:00
we differentiate between these
sodium and potassium ions.

446
00:21:00 --> 00:21:04
And the answer is just really
beautifully simple, and it's

447
00:21:04 --> 00:21:07
just that the pore gets really,
really tiny, to the point that

448
00:21:07 --> 00:21:10
it gets so small that all that
can fit through this pore is

449
00:21:10 --> 00:21:17
one a single ion, one single
sodium ion, solvated by one

450
00:21:17 --> 00:21:18
single water molecule.
 

451
00:21:18 --> 00:21:21
And that's all that's big
enough to pass through or

452
00:21:21 --> 00:21:23
small enough to pass through.
 

453
00:21:23 --> 00:21:26
And if we go up even just one
row on the periodic table to

454
00:21:26 --> 00:21:30
potassium, what we actually see
is now that it's going to be

455
00:21:30 --> 00:21:33
too large, and, in fact, a
potassium solvated with one

456
00:21:33 --> 00:21:36
water molecule won't go
through our channel.

457
00:21:36 --> 00:21:39
So, this is just one example of
how these properties can

458
00:21:39 --> 00:21:42
already, even our understanding
just talking about single

459
00:21:42 --> 00:21:44
atoms, can already make
an impact in these

460
00:21:44 --> 00:21:46
biological systems.
 

461
00:21:46 --> 00:21:48
And actually, a question that
might come up, I just

462
00:21:48 --> 00:21:51
explained, the sodium channel,
you might say, well, how do

463
00:21:51 --> 00:21:54
potassium channels work then,
because I can understand how

464
00:21:54 --> 00:21:56
you can filter something big
out, but how do you filter

465
00:21:56 --> 00:21:57
out something small.
 

466
00:21:57 --> 00:22:01
And it turns out that size
exclusion is also the principle

467
00:22:01 --> 00:22:04
that's in play with potassium
channels as well, but in this

468
00:22:04 --> 00:22:07
case it's a little more
complex, because what happens

469
00:22:07 --> 00:22:11
is these negative residues the
are in the pore need to

470
00:22:11 --> 00:22:13
stabilize the potassium as it
goes through, and the potassium

471
00:22:13 --> 00:22:18
is large enough to make all the
contacts it needs, but the

472
00:22:18 --> 00:22:21
sodium, which you can picture
being smaller, actually can't

473
00:22:21 --> 00:22:24
reach all of the stabilizing
charges that it needs to

474
00:22:24 --> 00:22:25
to get through the pore.
 

475
00:22:25 --> 00:22:28
So, again, it is based on size,
it's a little bit less

476
00:22:28 --> 00:22:30
intuitive than the idea of just
straining out all of

477
00:22:30 --> 00:22:31
the potassium ions.
 

478
00:22:31 --> 00:22:34
But again, many of these ion
channels have this size

479
00:22:34 --> 00:22:37
exclusion pore, it's a very
important part of them.

480
00:22:37 --> 00:22:41
All right, so that was a quick
aside on thinking about how

481
00:22:41 --> 00:22:44
these properties can, in fact,
relate to something

482
00:22:44 --> 00:22:45
in our body.
 

483
00:22:45 --> 00:22:49
Let's move on to the last topic
in terms of this first exam,

484
00:22:49 --> 00:22:51
which is thinking about the
idea of isoelectronic atoms,

485
00:22:51 --> 00:22:56
or isoelectronic ions.
 

486
00:22:56 --> 00:22:58
And isoelectronic is very
straightforward, it just

487
00:22:58 --> 00:23:01
means having the same
electron configuration.

488
00:23:01 --> 00:23:04
The easiest way to look at this
is just to do an example.

489
00:23:04 --> 00:23:07
So let's take the
example of neon.

490
00:23:07 --> 00:23:09
This has the electron
configuration of 1 s

491
00:23:09 --> 00:23:12
2, 2 s 2, and 2 p 6.
 

492
00:23:12 --> 00:23:16
It looks like we're cut off the
screen a little bit here, but

493
00:23:16 --> 00:23:18
you can see I've just
circled it there.

494
00:23:18 --> 00:23:21
So we can go ahead and think
about, well, are there any

495
00:23:21 --> 00:23:23
other atoms that are going to
have the same electron

496
00:23:23 --> 00:23:23
configuration?
 

497
00:23:23 --> 00:23:25
The answer to that is
definitely no -- if they

498
00:23:25 --> 00:23:27
had the same electron
configuration, they

499
00:23:27 --> 00:23:29
would, in fact, be neon.
 

500
00:23:29 --> 00:23:31
But we can think about
different ions that have this

501
00:23:31 --> 00:23:33
electron configuration.
 

502
00:23:33 --> 00:23:36
So for example, if we think
about fluorine, that has an

503
00:23:36 --> 00:23:41
electron configuration of 1 s
2, 2 s 2, 2 p 5, so all we

504
00:23:41 --> 00:23:44
would need to do is add one
more electron to get the same

505
00:23:44 --> 00:23:46
configuration as for neon.
 

506
00:23:46 --> 00:23:49
So if we want to write out what
that would be, it would just be

507
00:23:49 --> 00:23:55
to say that f minus is
isoelectronic with neon.

508
00:23:55 --> 00:24:00
So, we can say that -- if we
have neon here and we want

509
00:24:00 --> 00:24:03
to think about what's
isoelectronic, f minus

510
00:24:03 --> 00:24:04
would be isoelectronic.
 

511
00:24:04 --> 00:24:10
We also have oxygen -- what
would the charge on oxygen be?

512
00:24:10 --> 00:24:10
Um-hmm, right.
 

513
00:24:10 --> 00:24:12
2 minus.
 

514
00:24:12 --> 00:24:15
Then also, nitrogen, 3 minus
-- these are all going to

515
00:24:15 --> 00:24:18
be isoelectronic with neon.
 

516
00:24:18 --> 00:24:20
We can go in the other
direction, so let's go to

517
00:24:20 --> 00:24:23
sodium, but we would need to
take away an electron to

518
00:24:23 --> 00:24:25
make it isoelectronic.
 

519
00:24:25 --> 00:24:30
So we would say sodium plus, or
magnesium 2 plus, we can just

520
00:24:30 --> 00:24:37
keep going -- aluminum 3 plus,
silicone 4 plus, and we can go

521
00:24:37 --> 00:24:40
on and on in either direction
all the way across and

522
00:24:40 --> 00:24:42
down the periodic table.
 

523
00:24:42 --> 00:24:45
So, that's the idea of
isoelectronic ions.

524
00:24:45 --> 00:24:48
These are all isoelectronic,
they all have the same

525
00:24:48 --> 00:24:50
electron configuration.
 

526
00:24:50 --> 00:24:53
And we can also think about
going back to atomic

527
00:24:53 --> 00:24:54
size for a second.
 

528
00:24:54 --> 00:24:58
What the relationship is
between these ions and

529
00:24:58 --> 00:24:59
their parent atoms.
 

530
00:24:59 --> 00:25:05
So, for example, if we think of
the fluorine minus case, would

531
00:25:05 --> 00:25:09
you expect fluorine minus to be
larger or smaller than

532
00:25:09 --> 00:25:11
neutral fluorine?
 

533
00:25:11 --> 00:25:12
Okay.
 

534
00:25:12 --> 00:25:14
I heard mostly larger, but a
little bit of a mix in there,

535
00:25:14 --> 00:25:17
and it turns out that
larger is correct.

536
00:25:17 --> 00:25:20
And we can think about why --
essentially we have fluorine

537
00:25:20 --> 00:25:22
and now we're adding
another electron.

538
00:25:22 --> 00:25:25
So you can picture that
fluorine is going to get

539
00:25:25 --> 00:25:27
larger in this case.
 

540
00:25:27 --> 00:25:30
And that would be true for all
of the negatively charged ions.

541
00:25:30 --> 00:25:32
So, by the same logic, that
means that all of our

542
00:25:32 --> 00:25:36
positively charged ions are, in
fact, going to be smaller in

543
00:25:36 --> 00:25:39
terms of radius, compared to
their neutral parents.

544
00:25:39 --> 00:25:42
Not only are we taking away an
electron here, but we're also

545
00:25:42 --> 00:25:46
going to decrease shielding, so
the electrons that are already

546
00:25:46 --> 00:25:48
in there are going to feel a
higher z effective and will be

547
00:25:48 --> 00:25:53
pulling and the atom will
be getting smaller.

548
00:25:53 --> 00:25:56
And this is just a picture
showing some of these

549
00:25:56 --> 00:25:57
sizes with their parent.
 

550
00:25:57 --> 00:26:01
So, for example, a lithium
here, you can see how lithium

551
00:26:01 --> 00:26:04
plus is smaller than
the actual lithium atom

552
00:26:04 --> 00:26:06
in its neutral state.
 

553
00:26:06 --> 00:26:10
Whereas for fluorine, fluorine
is smaller than f minus is the

554
00:26:10 --> 00:26:13
one that's the outer
shell shown here.

555
00:26:13 --> 00:26:19
So, let's do a clicker question
on isoelectronic atoms.

556
00:26:19 --> 00:26:22
And now we're asking you
to look at krypton, so

557
00:26:22 --> 00:26:24
the atomic mass is 36.
 

558
00:26:24 --> 00:26:27
You can actually just grab that
handout, the second handout on

559
00:26:27 --> 00:26:29
the exam and look at the
periodic table there.

560
00:26:29 --> 00:26:32
So, which of the
following ions listed is

561
00:26:32 --> 00:26:43
isoelectronic with krypton?
 

562
00:26:43 --> 00:26:58
OK, let's take 10
seconds on that.

563
00:26:58 --> 00:26:58
OK, good.
 

564
00:26:58 --> 00:27:03
This might be our all-time
high, 89% got this right.

565
00:27:03 --> 00:27:04
This is great.
 

566
00:27:04 --> 00:27:07
So, selenium 2 minus is what's
going to be isoelectronic,

567
00:27:07 --> 00:27:10
because if you add two
electrons to selenium, you'll

568
00:27:10 --> 00:27:13
get the same electron
configuration that you

569
00:27:13 --> 00:27:14
have for krypton here.
 

570
00:27:14 --> 00:27:18
OK, I think we can safely
go back to notes.

571
00:27:18 --> 00:27:20
So, I said I would announce
it, that's the end

572
00:27:20 --> 00:27:22
of exam 1 material.
 

573
00:27:22 --> 00:27:24
So, if you compartmentalize
things in your brain in certain

574
00:27:24 --> 00:27:28
ways, put that off into the end
of the exam 1 part of your

575
00:27:28 --> 00:27:31
brain, and now we're going
to move on to exam 2.

576
00:27:31 --> 00:27:33
Remember, for exam 2, you still
need to know and understand

577
00:27:33 --> 00:27:37
everything you learned in exam
1, but you can put off learning

578
00:27:37 --> 00:27:41
it completely until we get
through, at least, next

579
00:27:41 --> 00:27:44
Wednesday before we start
maybe spending time on these

580
00:27:44 --> 00:27:47
concepts outside of class.
 

581
00:27:47 --> 00:27:51
So, we're going to start with
talking about bonding, and any

582
00:27:51 --> 00:27:54
time we have a chemical bond,
basically what we're talking

583
00:27:54 --> 00:27:57
about is having two atoms where
the arrangement of their nuclei

584
00:27:57 --> 00:28:01
and they're electrons are such
that the bonded atoms results

585
00:28:01 --> 00:28:04
in a lower energy than
for the separate atoms.

586
00:28:04 --> 00:28:07
So we know we always want to
have our systems in as low an

587
00:28:07 --> 00:28:11
energy as possible, so it makes
sense that a bond would happen

588
00:28:11 --> 00:28:14
any time we got a lower energy
when we combine two atoms,

589
00:28:14 --> 00:28:18
versus when we keep
them separate.

590
00:28:18 --> 00:28:20
So specifically, today
we're going to talk

591
00:28:20 --> 00:28:22
about covalent bonds.
 

592
00:28:22 --> 00:28:25
A covalent bond is any time we
have a pair of electrons that

593
00:28:25 --> 00:28:27
is shared between two
different atoms.

594
00:28:27 --> 00:28:30
And the key word for
covalent bonds is the

595
00:28:30 --> 00:28:31
idea of being shared.
 

596
00:28:31 --> 00:28:37
The two electrons, for example,
we see in the h 2 molecule,

597
00:28:37 --> 00:28:39
they don't belong to one or the
other atom, they're

598
00:28:39 --> 00:28:41
actually shared.
 

599
00:28:41 --> 00:28:44
And what we'll see later, is
that the sharing is not always

600
00:28:44 --> 00:28:47
equal -- in the case of h 2, it
is completely equal sharing.

601
00:28:47 --> 00:28:50
In some cases, because of
things like electronegativity,

602
00:28:50 --> 00:28:54
one atom will take away more of
the electron density than the

603
00:28:54 --> 00:28:56
other atom, but they're still
shared, even if they're

604
00:28:56 --> 00:28:58
not always evenly shared.
 

605
00:28:58 --> 00:29:01
So, in talking about covalent
bonds, we should be able to

606
00:29:01 --> 00:29:04
still apply a more general
definition of a chemical bond,

607
00:29:04 --> 00:29:08
which should tell us that the h
2 molecule is going to be lower

608
00:29:08 --> 00:29:11
in energy than if we looked at
2 separate hydrogen

609
00:29:11 --> 00:29:12
atom molecules.
 

610
00:29:12 --> 00:29:15
So, let's see if that's
actually the case.

611
00:29:15 --> 00:29:18
So if I tell you that the
energy for single hydrogen

612
00:29:18 --> 00:29:22
atom is negative 13 12
kilojoules per mole.

613
00:29:22 --> 00:29:25
If we want to talk about two
hydrogen atoms, then we just

614
00:29:25 --> 00:29:29
need to double that, so that's
going to be negative 2 6 2 4

615
00:29:29 --> 00:29:32
kilojoules per mole that we're
talking about in terms of

616
00:29:32 --> 00:29:33
a single hydrogen atom.
 

617
00:29:33 --> 00:29:38
So, let's compare this to the
energy of the h 2 molecule, and

618
00:29:38 --> 00:29:40
we find that that's negative
3,048 kilojoules per mole.

619
00:29:40 --> 00:29:45
So, in fact, yes, we did
confirm that these covalent

620
00:29:45 --> 00:29:48
bond, at least in the case of
hydrogen, we have confirmed by

621
00:29:48 --> 00:29:51
the numbers that we are at a
lower energy state when we

622
00:29:51 --> 00:29:55
talk about the bonded atom
versus the individual atom.

623
00:29:55 --> 00:29:59
And when we talk about covalent
bonds, there's 2 properties

624
00:29:59 --> 00:30:01
that we'll mostly focus on, and
that's going to be thinking

625
00:30:01 --> 00:30:05
about the bond strength or
the energy by which it

626
00:30:05 --> 00:30:07
stabilized when it bonds.
 

627
00:30:07 --> 00:30:10
And we can also talk about the
bond length, so we might be

628
00:30:10 --> 00:30:13
interested in what the bond
length is, what the distance

629
00:30:13 --> 00:30:15
between these two nuclei are.
 

630
00:30:15 --> 00:30:18
And we can actually better
visualize this if we plot

631
00:30:18 --> 00:30:21
how that energy changes
as a function of

632
00:30:21 --> 00:30:24
internuclear distance.
 

633
00:30:24 --> 00:30:26
And when I say internuclear
distance, we actually

634
00:30:26 --> 00:30:28
call this r here.
 

635
00:30:28 --> 00:30:31
It's kind of ironic that we put
this in the same lecture as we

636
00:30:31 --> 00:30:34
talk about atomic radii, which
we also call r, but they're two

637
00:30:34 --> 00:30:37
different r's, so you need to
keep them separated in terms

638
00:30:37 --> 00:30:38
of what you're talking about.
 

639
00:30:38 --> 00:30:42
When we're talking about r for
internuclear distance, we're

640
00:30:42 --> 00:30:45
talking about the distance
between two different nuclei in

641
00:30:45 --> 00:30:48
a bond, in a covalent bond.
 

642
00:30:48 --> 00:30:52
So, if we look at this graph
where what we're charting is

643
00:30:52 --> 00:30:54
the internuclear distance, so
the distance between these two

644
00:30:54 --> 00:30:58
hydrogen atoms, as a function
of energy, what we are going to

645
00:30:58 --> 00:31:01
see is a curve that looks like
this -- this is the general

646
00:31:01 --> 00:31:04
curve that you'll see for any
covalent bond, and we'll

647
00:31:04 --> 00:31:07
explain where that comes
from in a minute.

648
00:31:07 --> 00:31:10
I want to point out that the
zero energy is defined as when

649
00:31:10 --> 00:31:15
you have a naked proton where
the electron has popped out --

650
00:31:15 --> 00:31:17
that's what we've defined as
zero energy up to this

651
00:31:17 --> 00:31:19
point when we're talking
about single atoms.

652
00:31:19 --> 00:31:22
So, for starters we'll keep
that as our zero energy, we're

653
00:31:22 --> 00:31:25
going to change it soon to make
something that makes more sense

654
00:31:25 --> 00:31:28
in terms of bonding, but we'll
keep that as zero for now.

655
00:31:28 --> 00:31:32
So, we see that the two h atoms
separate have a certain energy

656
00:31:32 --> 00:31:36
that's lower than when the
electron's not with the atom.

657
00:31:36 --> 00:31:39
And then even lower down,
we have our bonded

658
00:31:39 --> 00:31:42
hydrogen molecule.
 

659
00:31:42 --> 00:31:44
So, we can think about the
different kinds of interactions

660
00:31:44 --> 00:31:45
that are taking place.
 

661
00:31:45 --> 00:31:48
I said what hold the bonds
together, what holds two atoms

662
00:31:48 --> 00:31:52
together is the attractive
force we have between each

663
00:31:52 --> 00:31:55
electron and the other nucleus.
 

664
00:31:55 --> 00:31:57
That's the huge force that
we're talking about in terms of

665
00:31:57 --> 00:32:02
making a bond stable, but there
are also repulsive forces, so

666
00:32:02 --> 00:32:04
you can imagine we're going to
have electron-electron

667
00:32:04 --> 00:32:07
repulsion between the two
electrons if we're bringing

668
00:32:07 --> 00:32:08
them closer together.
 

669
00:32:08 --> 00:32:11
And the real killer is if we
get too close we're even going

670
00:32:11 --> 00:32:13
to have nuclear-nuclear
repulsion between the

671
00:32:13 --> 00:32:16
nuclei of the two atoms.
 

672
00:32:16 --> 00:32:19
So, this makes this chart shown
in pink make a lot more sense,

673
00:32:19 --> 00:32:23
because if we're way out at
very far distances, essentially

674
00:32:23 --> 00:32:25
what we have here is we're
talking about two

675
00:32:25 --> 00:32:26
separate atoms.
 

676
00:32:26 --> 00:32:29
They're not interacting at all
so that's why the energy is

677
00:32:29 --> 00:32:32
the same as that for two
individual atoms, that's

678
00:32:32 --> 00:32:33
what we're dealing with.
 

679
00:32:33 --> 00:32:36
As we get closer together,
we start get lower

680
00:32:36 --> 00:32:38
and lower in energy.
 

681
00:32:38 --> 00:32:40
The reason is because the
predominant force at this point

682
00:32:40 --> 00:32:44
is going to be the attraction
that's being felt between the

683
00:32:44 --> 00:32:47
nuclei and the electrons
in each of the atoms.

684
00:32:47 --> 00:32:51
At some point you're going to
hit a well here, which is the

685
00:32:51 --> 00:32:54
point where it's most
stabilized or at

686
00:32:54 --> 00:32:56
it's lowest energy.
 

687
00:32:56 --> 00:32:58
So, when we think about a bond
length, this is going to be the

688
00:32:58 --> 00:33:01
length of our bond here, that
makes sense because it's going

689
00:33:01 --> 00:33:05
to want to be at that distance
that minimizes the energy.

690
00:33:05 --> 00:33:08
But as we keep getting closer,
even though as we get closer,

691
00:33:08 --> 00:33:10
the attraction is going to
get stronger between the two

692
00:33:10 --> 00:33:11
nuclei and the electrons.
 

693
00:33:11 --> 00:33:14
We're also going to start to
have the repulsive forces

694
00:33:14 --> 00:33:17
become more prominent here,
and, in fact, they take over at

695
00:33:17 --> 00:33:21
some point, becoming the more
prevalent of the forces, so as

696
00:33:21 --> 00:33:22
you get closer, the
electron-electron repulsions,

697
00:33:22 --> 00:33:26
and eventually the
nucleus-nucleus repulsion is

698
00:33:26 --> 00:33:29
going to mean that your energy
is just absolutely

699
00:33:29 --> 00:33:32
skyrocketing, so it just keeps
going up and up as you get

700
00:33:32 --> 00:33:36
closer to zero here.
 

701
00:33:36 --> 00:33:39
So, when we want to talk about
the information that we can get

702
00:33:39 --> 00:33:41
out of looking at a chart like
this, well, the first thing I

703
00:33:41 --> 00:33:44
did tell you was that this is
going to be the bond length, so

704
00:33:44 --> 00:33:47
the distance r where the energy
is lowest, but we can also talk

705
00:33:47 --> 00:33:51
about something called
dissociation energy, that's

706
00:33:51 --> 00:33:54
going to be this distance
right here or the energy

707
00:33:54 --> 00:33:56
that is this value.
 

708
00:33:56 --> 00:33:59
And the dissociation energy is
very intuitive in terms of what

709
00:33:59 --> 00:34:02
it means, it means how much
energy you need to put into the

710
00:34:02 --> 00:34:05
molecule in order to
disassociate it into

711
00:34:05 --> 00:34:07
its individual atoms.
 

712
00:34:07 --> 00:34:10
And so we can actually think
about how do we calculate

713
00:34:10 --> 00:34:13
what the dissociation energy
should be for h 2, so let's

714
00:34:13 --> 00:34:16
go ahead and do this.
 

715
00:34:16 --> 00:34:20
So, if we talk about
dissociating h 2, we're going

716
00:34:20 --> 00:34:25
from the h 2 molecule, and
breaking this bond right in

717
00:34:25 --> 00:34:31
half, so we now have two
individual hydrogen atoms here.

718
00:34:31 --> 00:34:35
So we need to take the energy
for the two atoms, which we

719
00:34:35 --> 00:34:39
know is -- so let's take our
dissociation energy is going to

720
00:34:39 --> 00:34:48
be equal to negative 2 6 2 4
kilojoules per mole, and we

721
00:34:48 --> 00:34:52
want to subtract the energy of
the hydrogen molecule itself,

722
00:34:52 --> 00:35:00
so that's going to be negative
3 0 4 8 kilojoules per mole.

723
00:35:00 --> 00:35:03
So, what we get for the
disassociation energy for

724
00:35:03 --> 00:35:11
a hydrogen atom is 424
kilojoules per mole.

725
00:35:11 --> 00:35:14
So what that means is that's
how much energy we would have

726
00:35:14 --> 00:35:17
to put in to a hydrogen
molecule in order to get it to

727
00:35:17 --> 00:35:25
split apart into its two atoms.
 

728
00:35:25 --> 00:35:28
So, another way to talk about
dissociation energy is simply

729
00:35:28 --> 00:35:31
to call it bond strength, it's
the same thing, they're

730
00:35:31 --> 00:35:32
equal to each other.
 

731
00:35:32 --> 00:35:36
If we know that this is it the
dissociation energy for a

732
00:35:36 --> 00:35:38
hydrogen atom, we can also
say the bond strength for

733
00:35:38 --> 00:35:43
hydrogen molecule is 424.
 

734
00:35:43 --> 00:35:45
So, there's actually another
way to graph it where we can

735
00:35:45 --> 00:35:49
directly graph the dissociation
energy or the bond strengths.

736
00:35:49 --> 00:35:53
So I said before when we were
talking about single atoms, we

737
00:35:53 --> 00:35:55
always define the zero energy
as when an electron was

738
00:35:55 --> 00:36:00
actually ejected, but now, when
we talk about chemical

739
00:36:00 --> 00:36:03
reactions taking place, it's
very, very rare that we're

740
00:36:03 --> 00:36:05
actually going to be talking
about anything that gets

741
00:36:05 --> 00:36:06
to this point here.
 

742
00:36:06 --> 00:36:10
It's much more relevant to set
our zero point energy as the

743
00:36:10 --> 00:36:14
separation of a bond in terms
of talking about the reactions

744
00:36:14 --> 00:36:16
that we'll usually be
dealing with here.

745
00:36:16 --> 00:36:20
So, let's change our graph
where we now have this zero

746
00:36:20 --> 00:36:24
point set as the two
individuals hydrogen atoms, and

747
00:36:24 --> 00:36:27
then we see that our h 2
molecule is at the negative of

748
00:36:27 --> 00:36:30
the dissociation energy, or the
negative what that

749
00:36:30 --> 00:36:31
bond strength is.
 

750
00:36:31 --> 00:36:33
So we know what that number
would be, it would be

751
00:36:33 --> 00:36:37
negative 424 kilojoules
per mole that we see here.

752
00:36:37 --> 00:36:41
So, what this let's us do now
is directly compare, for

753
00:36:41 --> 00:36:46
example, the strength of a bond
in terms of a hydrogen atom and

754
00:36:46 --> 00:36:49
hydrogen molecule, compared to
any kind of molecule that we

755
00:36:49 --> 00:36:51
want to graph on top of it.
 

756
00:36:51 --> 00:36:54
So, let's, for example,
look at nitrogen.

757
00:36:54 --> 00:36:58
So n 2, we can do the chart
here in green, so it's the

758
00:36:58 --> 00:37:02
green dotted line, and what we
see is that we have now defined

759
00:37:02 --> 00:37:05
this energy as where the 2
nitrogen atoms are separated.

760
00:37:05 --> 00:37:08
So what we can actually
directly compare is the

761
00:37:08 --> 00:37:12
dissociation energy or the
bond strength of nitrogen

762
00:37:12 --> 00:37:14
versus hydrogen.
 

763
00:37:14 --> 00:37:16
So, if we think about
this, which would you

764
00:37:16 --> 00:37:17
say has a stronger bond?
 

765
00:37:17 --> 00:37:21
Is it going to be
hydrogen or nitrogen?

766
00:37:21 --> 00:37:23
Yup, it's going to be nitrogen.
 

767
00:37:23 --> 00:37:26
And the reason we can see that
by looking at this graph is

768
00:37:26 --> 00:37:29
that we see that nitrogen when
it's bonded is in an even lower

769
00:37:29 --> 00:37:31
well than we saw for hydrogen.
 

770
00:37:31 --> 00:37:33
It's going to be a stronger
bond because it's more

771
00:37:33 --> 00:37:38
stabilized when it when it
comes together as a molecule.

772
00:37:38 --> 00:37:42
We can also think about the
distance, the bond distance.

773
00:37:42 --> 00:37:43
So, which would you
say is going to be

774
00:37:43 --> 00:37:44
shorter in this case?
 

775
00:37:44 --> 00:37:47
Is a hydrogen bond shorter, or
is a nitrogen-nitrogen triple

776
00:37:47 --> 00:37:50
bond going to be shorter?
 

777
00:37:50 --> 00:37:52
Um-hmm, again, we can
get this information

778
00:37:52 --> 00:37:54
directly from our graph.
 

779
00:37:54 --> 00:37:57
We see that the radius is
shorter, so that means that

780
00:37:57 --> 00:38:00
the nitrogen-nitrogen bond
is going to be shorter.

781
00:38:00 --> 00:38:03
We can know this information
even if we just knew that the

782
00:38:03 --> 00:38:05
bond was stronger, we wouldn't
need to look at a graph here,

783
00:38:05 --> 00:38:08
because it turns out that if
you have a stronger bond, that

784
00:38:08 --> 00:38:11
also means that you have a
shorter bond -- those

785
00:38:11 --> 00:38:12
ywo are correlated.
 

786
00:38:12 --> 00:38:15
And something that we'll see
later on is that triple bonds,

787
00:38:15 --> 00:38:18
for example, are going to be
stronger than a corresponding

788
00:38:18 --> 00:38:21
double bond or a
corresponding single bond.

789
00:38:21 --> 00:38:24
So, if we talked about a
nitrogen-nitrogen single versus

790
00:38:24 --> 00:38:26
double versus triple bond, the
triple bond will be the

791
00:38:26 --> 00:38:29
shortest and it will
be the strongest.

792
00:38:29 --> 00:38:32
So, that's basically the idea
of how we are going to be

793
00:38:32 --> 00:38:33
thinking about covalent bonds.
 

794
00:38:33 --> 00:38:37
It's also important, once we
start talking about molecules,

795
00:38:37 --> 00:38:39
to have a way to represent
them, and also to be able to

796
00:38:39 --> 00:38:43
look at a shorthand notation
for a certain molecule and

797
00:38:43 --> 00:38:45
understand what the bond is.
 

798
00:38:45 --> 00:38:48
So, for example, down here I
wrote that it was n 2 and that

799
00:38:48 --> 00:38:52
it was h 2, but when I re-wrote
the molecules up here, you saw

800
00:38:52 --> 00:38:55
that it's an h h single bond
where it's a nitrogen-nitrogen

801
00:38:55 --> 00:38:56
triple bond.
 

802
00:38:56 --> 00:39:00
So any chemist should be able
to just look at n 2 and know

803
00:39:00 --> 00:39:02
that it's a triple bond, but
that's not something that we've

804
00:39:02 --> 00:39:05
learned how did to do yet, so
let's go ahead and start a new

805
00:39:05 --> 00:39:08
topic that's going to allow us
to have some sort of sense of

806
00:39:08 --> 00:39:11
what the valence electron
configuration, which includes

807
00:39:11 --> 00:39:14
whether something's a single or
double or a triple bond can be

808
00:39:14 --> 00:39:17
figured out for any
given molecule.

809
00:39:17 --> 00:39:19
So, to do this, what I'm
going to do is introduce the

810
00:39:19 --> 00:39:21
topic of Lewis structures.
 

811
00:39:21 --> 00:39:24
We're going to really get into
this next class, but I just

812
00:39:24 --> 00:39:27
want to introduce it to you to
give us a start, and many of

813
00:39:27 --> 00:39:30
you have used Lewis structures
in high school, but we'll be

814
00:39:30 --> 00:39:33
doing some much more
challenging Lewis structures, I

815
00:39:33 --> 00:39:37
can assure you, in
this class here.

816
00:39:37 --> 00:39:41
So, a Lewis structure is
basically an organizing

817
00:39:41 --> 00:39:46
property of bonding, of
molecules, which is the idea

818
00:39:46 --> 00:39:49
that when we're thinking about
bonding, the key is to achieve

819
00:39:49 --> 00:39:53
a full valence shell in each
of the individual atoms.

820
00:39:53 --> 00:39:57
So we want to have in an h h
bond, for example, a full shell

821
00:39:57 --> 00:40:00
for each of the hydrogen atoms.
 

822
00:40:00 --> 00:40:00
And G.N.
 

823
00:40:00 --> 00:40:04
Lewis is the scientist that is
credited, and who did, in fact,

824
00:40:04 --> 00:40:08
come up with this idea for the
way to represent this, so the

825
00:40:08 --> 00:40:11
other parts of this idea,
another way to phrase it is

826
00:40:11 --> 00:40:14
that the electrons are going to
be distributed in such a way

827
00:40:14 --> 00:40:17
that we have what are called
full octets for each of the

828
00:40:17 --> 00:40:21
atoms, and basically that's the
same thing as saying we have a

829
00:40:21 --> 00:40:24
full valence shell, and this is
something that Lewis was able

830
00:40:24 --> 00:40:28
to recognize very, very early,
way before we had quantum

831
00:40:28 --> 00:40:31
mechanics to describe what
these orbitals were, but it

832
00:40:31 --> 00:40:35
makes sense a full valence
shell means for most atoms that

833
00:40:35 --> 00:40:40
we have a full s orbital plus a
full p orbital, so we're going

834
00:40:40 --> 00:40:42
to have a total of four
orbitals that are each filled

835
00:40:42 --> 00:40:45
with eight electrons, so
that's why we see that

836
00:40:45 --> 00:40:48
we need an octet here.
 

837
00:40:48 --> 00:40:52
And the idea is that when you
do these Lewis dot structures,

838
00:40:52 --> 00:40:55
we're representing electrons
with dots, which we'll see in a

839
00:40:55 --> 00:40:58
minute, and each dot
is going to represent

840
00:40:58 --> 00:40:59
a valence electron.
 

841
00:40:59 --> 00:41:02
So, hopefully, you remember
what we mean by valence

842
00:41:02 --> 00:41:04
electrons versus
core electrons.

843
00:41:04 --> 00:41:07
Core electrons are all those
electrons held in really tight

844
00:41:07 --> 00:41:10
with the nucleus in the inner
shells, whereas the valence

845
00:41:10 --> 00:41:12
electrons are only those
electrons that are in the

846
00:41:12 --> 00:41:16
outer-most shell, or at your
highest value of n of the

847
00:41:16 --> 00:41:19
principal quantum number.
 

848
00:41:19 --> 00:41:23
So, Lewis structures are really
a model for a way to think

849
00:41:23 --> 00:41:26
about what the valence electron
configuration is, and as I

850
00:41:26 --> 00:41:29
said, it's not based on quantum
mechanics, it's something that

851
00:41:29 --> 00:41:33
Lewis observed far, far before
quantum mechanics

852
00:41:33 --> 00:41:34
were discovered.
 

853
00:41:34 --> 00:41:37
So he came up with the
ideas that led to the idea

854
00:41:37 --> 00:41:41
of Lewis structures in
the very early 1900's.

855
00:41:41 --> 00:41:44
So you might ask well, why are
we using this model if it

856
00:41:44 --> 00:41:47
clearly doesn't take into
account quantum mechanics?

857
00:41:47 --> 00:41:49
And the reason that we use it
is that it is incredibly

858
00:41:49 --> 00:41:52
accurate, and allows us to
very, very quickly predict and

859
00:41:52 --> 00:41:55
to predict accurately, in most
cases, what the electron

860
00:41:55 --> 00:41:58
configuration of molecules
are going to be.

861
00:41:58 --> 00:42:00
So this is really useful.
 

862
00:42:00 --> 00:42:03
We don't always want to go and
solve the Schrodinger equation,

863
00:42:03 --> 00:42:06
and in fact, once we start
talking about molecules, I can

864
00:42:06 --> 00:42:08
imagine none of you, as much as
you love math or physics, want

865
00:42:08 --> 00:42:10
to be trying to solve this
Schrodinger equation

866
00:42:10 --> 00:42:12
in that case either.
 

867
00:42:12 --> 00:42:15
So, what Lewis structures allow
us to do is over 90% of the

868
00:42:15 --> 00:42:18
time be correct in terms
of figuring out what the

869
00:42:18 --> 00:42:20
electron configuration is.
 

870
00:42:20 --> 00:42:22
And we won't just use
them in this class.

871
00:42:22 --> 00:42:25
If you actually go to any of
the chemistry labs at MIT, if

872
00:42:25 --> 00:42:28
you go over to building 18 and
look in the organic labs where

873
00:42:28 --> 00:42:31
they're synthesizing new
molecules or making up new

874
00:42:31 --> 00:42:35
reactions, what you'll see if
you open anyone's notebook,

875
00:42:35 --> 00:42:39
their lab notebook, assuming
they keep a nice lab notebook,

876
00:42:39 --> 00:42:42
is that they will have Lewis
structures drawn in there that

877
00:42:42 --> 00:42:44
explain the reactions that
they're going to be

878
00:42:44 --> 00:42:45
doing for that day.
 

879
00:42:45 --> 00:42:48
And I mean this means way past
all the chemistry they've

880
00:42:48 --> 00:42:51
taken, they're now graduate
students or they're now

881
00:42:51 --> 00:42:53
professors, and they're still
writing out Lewis structures.

882
00:42:53 --> 00:42:56
Now they're writing a more
abbreviated form, which you'll

883
00:42:56 --> 00:42:59
probably get to if you take
organic chemistry, but really

884
00:42:59 --> 00:43:02
it's the exact same idea.
 

885
00:43:02 --> 00:43:05
And this goes all the
way back to 1902.

886
00:43:05 --> 00:43:09
In fact, Lewis was an American
scientist, so he was trained in

887
00:43:09 --> 00:43:14
America, and he actually was a
professor here at MIT from 1905

888
00:43:14 --> 00:43:19
all the way to about 1911 or
1912, and these are some notes

889
00:43:19 --> 00:43:23
from 1902, and you can't see
them very well, but this was

890
00:43:23 --> 00:43:27
essentially an early form of
Lewis structures, and this

891
00:43:27 --> 00:43:29
was called the cubicle atom.
 

892
00:43:29 --> 00:43:32
So, basically what he's showing
in these cubes is that there

893
00:43:32 --> 00:43:34
are eight spaces that need
to be filled up to

894
00:43:34 --> 00:43:35
have a full cube.
 

895
00:43:35 --> 00:43:38
So in order to fill them, he
would have to have eight

896
00:43:38 --> 00:43:41
electrons or an octet
around the cubes.

897
00:43:41 --> 00:43:45
So what we're seeing is this is
notes from 1902 -- he actually

898
00:43:45 --> 00:43:49
didn't publish any of this work
or these ideas that led to

899
00:43:49 --> 00:43:52
Lewis structures until 1916,
but his early class notes were

900
00:43:52 --> 00:43:55
used as evidence about how long
ago he actually came up

901
00:43:55 --> 00:43:57
with the idea of it.
 

902
00:43:57 --> 00:43:59
So it's really neat to think
that your counterparts 100

903
00:43:59 --> 00:44:02
years ago right here at MIT
could have been sitting in a

904
00:44:02 --> 00:44:05
class where they had Lewis as
their lecturer, and he's

905
00:44:05 --> 00:44:07
putting forth these ideas --
these are actually his lecture

906
00:44:07 --> 00:44:11
notes, even though it wasn't
even published yet, and giving

907
00:44:11 --> 00:44:13
this idea of Lewis structure,
which is exactly what we keep

908
00:44:13 --> 00:44:16
using today in order to make a
lot of these predictions.

909
00:44:16 --> 00:44:19
So, let's see how some of this
works, and hopefully your

910
00:44:19 --> 00:44:22
counterparts from 100 years ago
would also be able to think

911
00:44:22 --> 00:44:26
about how this works, even if
they don't have the quantum

912
00:44:26 --> 00:44:30
mechanics behind the individual
electron configurations

913
00:44:30 --> 00:44:31
for atoms.
 

914
00:44:31 --> 00:44:34
So I said that we want to be
talking about valence electrons

915
00:44:34 --> 00:44:37
here, so that means if we're
talking about, for example, the

916
00:44:37 --> 00:44:41
octet rule for an f f molecule
where we have two fluorine

917
00:44:41 --> 00:44:43
atoms, we need to write the
valence electrons as

918
00:44:43 --> 00:44:45
dots around them.
 

919
00:44:45 --> 00:44:48
So let's do a quick clicker
question, and you tell me

920
00:44:48 --> 00:44:52
how many valence electrons
does fluorine have?

921
00:44:52 --> 00:44:55
Remember, valence electrons are
different from core, they're

922
00:44:55 --> 00:45:03
only the outer-most electrons
in the outer-most shell.

923
00:45:03 --> 00:45:16
So, 10 seconds on this,
this should be fast.

924
00:45:16 --> 00:45:17
OK, great.
 

925
00:45:17 --> 00:45:19
Good job on the clicker
questions today.

926
00:45:19 --> 00:45:21
So we have seven
valence electrons.

927
00:45:21 --> 00:45:24
So, let's go back to the
notes, and let's fill

928
00:45:24 --> 00:45:26
these in, seven electrons.
 

929
00:45:26 --> 00:45:28
Another way you could have
known them was to look at

930
00:45:28 --> 00:45:32
Lewis' notes here, where if
look at this box carefully you

931
00:45:32 --> 00:45:35
see there are seven dots around
the cube, so there are his

932
00:45:35 --> 00:45:38
seven valence electrons.
 

933
00:45:38 --> 00:45:41
So, we see is when we use the
octet rule to look at fluorine

934
00:45:41 --> 00:45:45
molecule, we're combining two
fluorine atoms, and what we end

935
00:45:45 --> 00:45:48
up with is an f f molecule
where they're sharing two

936
00:45:48 --> 00:45:51
electrons, so making
that covalent bond.

937
00:45:51 --> 00:45:54
But that each individual
fluorine atom has eight

938
00:45:54 --> 00:45:56
electrons, or full
octet around it.

939
00:45:56 --> 00:45:59
We can think about where those
electrons came from, so we got

940
00:45:59 --> 00:46:03
seven from the blue electrons
here, seven as shown in green

941
00:46:03 --> 00:46:07
here, but each individual
fluorine atom has eight, even

942
00:46:07 --> 00:46:10
though two of those are being
shared between both of them.

943
00:46:10 --> 00:46:13
So, the octet rule is a
general rule that you'll

944
00:46:13 --> 00:46:15
for all of the atoms.
 

945
00:46:15 --> 00:46:17
There are some exceptions,
which we'll get to later, but

946
00:46:17 --> 00:46:21
the only a big exception here
is with hydrogen, which has

947
00:46:21 --> 00:46:24
a special stability that's
associated with two electrons.

948
00:46:24 --> 00:46:26
This should make a lot of
sense, because we know that a

949
00:46:26 --> 00:46:32
hydrogen has 1 s as it's
outer-most or valence orbital,

950
00:46:32 --> 00:46:37
so it can be filled up just
with two 1 s electrons.

951
00:46:37 --> 00:46:40
And we give different names,
depending on what kind of

952
00:46:40 --> 00:46:43
electrons we're dealing with,
so, for example, with h c l

953
00:46:43 --> 00:46:46
here, we can talk about having
bonded versus lone

954
00:46:46 --> 00:46:48
pair electrons.
 

955
00:46:48 --> 00:46:50
So, in terms of the c l
atom, we need to talk about

956
00:46:50 --> 00:46:52
each atom individually.
 

957
00:46:52 --> 00:47:01
How many bonding
electrons does c l have?

958
00:47:01 --> 00:47:01
All right.
 

959
00:47:01 --> 00:47:05
Let's see, we've got a mixed
response here, it turns out it

960
00:47:05 --> 00:47:06
has two bonding electrons.
 

961
00:47:06 --> 00:47:08
I heard some people say
one, and that's a good

962
00:47:08 --> 00:47:11
guess, remember they're
actually sharing.

963
00:47:11 --> 00:47:14
So these two electrons, they
belong to chlorine, they also

964
00:47:14 --> 00:47:17
belong to hydrogen, but
they do, in fact, belong

965
00:47:17 --> 00:47:17
to chlorine as well.
 

966
00:47:17 --> 00:47:20
There's no one person owning
them, so they both have

967
00:47:20 --> 00:47:22
two electrons here
that are bonding.

968
00:47:22 --> 00:47:26
So how many lone pair
electrons do we have?

969
00:47:26 --> 00:47:26
OK.
 

970
00:47:26 --> 00:47:29
I hear six and three, so both
are sort of right, we have 6

971
00:47:29 --> 00:47:31
lone pair electrons, which
means that we have

972
00:47:31 --> 00:47:35
three lone pairs.
 

973
00:47:35 --> 00:47:39
So, in terms of thinking about
how to draw a Lewis structure,

974
00:47:39 --> 00:47:42
I won't go through this today
or any day in terms of just

975
00:47:42 --> 00:47:45
reading through the rules,
you can read that yourself.

976
00:47:45 --> 00:47:47
But what we'll do is go through
each of these rules in

977
00:47:47 --> 00:47:48
terms of an example.
 

978
00:47:48 --> 00:47:52
So, what will start with on
Monday is doing the most simple

979
00:47:52 --> 00:47:56
example of methane using
these Lewis structure rules.

980
00:47:56 --> 00:47:59
So, don't forget to study this
weekend and get those extra

981
00:47:59 --> 00:48:02
practice problems from
the course website.

982
00:48:02 --> 00:48:03