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Today is a very big day for all
of you.

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Your first semester at MIT is
half over.

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And, in addition to that,
you have a lot of challenges

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remaining ahead of you for the
rest of this semester.

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I am going to help make the
chemical challenges fun for you.

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That is what I am here to do.
I love chemistry.

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I love teaching 5.112.
This is the best part of my

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week.
So, here we are.

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Do you guys know that chemistry
can be incredibly fun?

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Let me just show you how fun
chemistry can be.

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And that is just the gravy.
The fun part is actually making

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the molecules.
Any of you see this picture in

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U.S.A.
Today recently?

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Not reading that important
journal, I see.

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Well, this picture did appear
very recently in U.S.A.

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Today in a story that talked
about the recent Nobel Prize in

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chemistry that was awarded to
Professor Richard R.

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Schrock, of our department.
And, in fact,

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I don't know if you realize
this, but Professor Schrock was

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the first person ever awarded
the Nobel Prize in Chemistry at

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MIT for work conducted while at
MIT.

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That is a pretty amazing
accomplishment because we have

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so many tremendous chemists on
the faculty here,

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and he is the first one to
break through in that manner.

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And maybe you are here at a
very special time,

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and we will see a number more
of these in the next few years.

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May with you contributing,
so much the better because you

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are here.
And let me just explain

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something, too.
Chemistry is fun.

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And only on very rare occasions
like this would Professor

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Schrock be drinking champagne at
8:30 in the morning,

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when this picture was taken.
The Reuters reporter,

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Brian, who took this photo,
is actually a pretty good

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friend of Professor Nocera here.
And so he rode over from the

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Schrock household.
And Dick and Dan came in

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together in the same car,
as they often do,

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from Winchester on that
morning, which was a very

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exciting day.
If you come to see me for

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office hours maybe I will tell
you more about it in detail,

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but it was an exciting day.
And I am in here,

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too.
My name is Kit Cummins.

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Kit is my nickname.
My name is Christopher.

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You can call me Christopher or
Kit, or you can call me

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Professor Cummins,
or you can just say hey,

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chemistry guy.
That will be fine.

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I am in this picture primarily
because it is part of being in

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the right place at the right
time, just like I am here now in

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front of you.
But also I am in it because we

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three are three in one of the
areas in our department that

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really focus our research on
inorganic chemistry.

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And what is inorganic
chemistry?

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I will say more about this
during the semester.

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But inorganic chemistry is
really the chemistry of the

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elements of the Periodic Table
inclusively.

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So, a pretty broad topic.
And I like doing chemistry with

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lots of elements.
And so that is why these guys

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and I share an association with
each other.

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And, in fact,
I was a graduate student with

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Professor Schrock.
And I am very,

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very proud to be able to say
that.

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Hopefully, my computer will be
staying on, here.

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And I want to point out,
too, that people who love

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science want to know something
about its history.

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And part of all of that is
knowing the academic family from

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which you derive,
depending on the person you've

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studied with in learning about
chemistry.

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And I studied with Schrock.
And Schrock studied with

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Osborn, a fantastic chemist in
his own right,

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no longer alive,
sadly, to see his progeny,

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Professor Schrock,
win the Nobel Prize.

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And Osborn studied with
Wilkinson, and Wilkinson was

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also a Nobel Prize winner.
So, you do not have to go very

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far back in that lineage to
encounter that recognition,

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the Nobel Prize,
Schrock, and then it skipped

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Osborne, and if you go back to
Wilkinson, there is another one.

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And you can keep going back.
And I am not going to go

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through that whole history today
because I have another history

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that I am going to need to go
through with you today.

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And I am going to bring this
back over so it does not

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distract you.
And this other history that I

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need to impart to you today has
to do with some of the

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contributions of Gilbert Newton
Lewis.

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This is one of the most famous
of all chemists ever.

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And if you like chemistry,
I encourage you to think of the

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word Newton, as it is found
engraved in one of the pillars

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in Killian Court at MIT as the
middle name of Gilbert Newton

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Lewis.
It is conceivable that having

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that middle name weighed heavily
upon Gilbert during his

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lifetime.
But it remains a good moniker.

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And in the future,
when you have kids,

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I encourage you to consider
using it as a middle name.

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It worked well in this
particular case.

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When you get the notes for
today, which incidentally will

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be available on the website
after class today,

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so you don't need to write down
everything I am writing down.

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You are actually going to get a
little bit more in the online

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version than what I am writing
down here.

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And if you find that you forgot
something that I said during

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class, just email me.
I will tell you what I said.

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It is not a problem.
But if you want to write down

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notes while I am speaking just
to make notes that you can refer

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to later to refresh your memory,
that is fine.

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I think your taking in
information in lots of different

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ways.
We are going to use the

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computer and we are going to use
the blackboard today.

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And you are going to have
available stuff from the

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Internet a little bit later,
too.

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And all of that is good.
I think you should try to

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reinforce and not worry if some
of the information is a little

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redundant.
And never be afraid to ask

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questions, especially,
I like to take questions by

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email or if you come knock on my
office door.

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In 1916, Gilbert Lewis
published a landmark paper in

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chemistry in the journal of the
American Chemical Society which

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remains the flagship journal of
the American Chemical Society.

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And this paper was entitled
"The Atom and the Molecule."

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Besides being an incredibly
important and influential paper

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in the history of chemistry,
"The Atom and the Molecule" was

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also the basis for --

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Like I said,
besides being a hugely

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influential manuscript,
"The Atom and the Molecule"

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also laid the foundations for
Lewis's thinking about

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electronic structure theory that
shaped the way we still today

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talk about valence and chemical
bonding.

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So, an incredibly important
paper from 1916.

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In the lecture notes today,
you will find the specific

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volume and page number for that
reference in the journal of the

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American Chemical Society.
And I encourage you in your

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copious free time to download
that paper.

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It is really great.
All the American Chemical

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Society journal articles are
available, from the very

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beginning.
And you can download it and

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read it.
And, having had this

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introduction today,
I think it will probably prove

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to be a very fascinating
exercise for you,

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if you choose to do so.
It also was the basis of a book

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that Gilbert Newton Lewis later
wrote on valance and on atoms

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and molecules.
That is also a classic in

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chemistry.
And, should you be shopping for

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chemistry books on e-bay or
something, you might find a rare

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copy of it.
And if you do and choose not to

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buy it, please alert me and I
will buy it.

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But Gilbert Newton Lewis,
"The Atom and the Molecule."

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In this paper you are going to
see, if you do choose to

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download it, that he was
thinking about polarity.

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And he was also thinking about
the relationship between the

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polarity of a molecule relates
to the polarity of a substance.

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And I mean, when I say a
polarity of a substance,

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that that would be a sample,
you could say a water molecule

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down here and liquid water up
here.

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And you will see this kind of
sigmoidal graph that Lewis made

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in thinking about how polar
condensed phase substances were

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as compared with the structure
of the molecules that comprise

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the substance.
Down here, in this region,

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you will find that there are
substances that are not terribly

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polar.
And what I have written here is

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the organic shorthand for the
hexane molecule.

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Liquid hexane is an alkane,
akin to methane,

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which is natural gas.
And so, if you are not familiar

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with this way of writing organic
molecules, I will just tell you

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briefly that this terminus is CH
three or a methyl group,

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and then it is CH two,
CH two, CH two,

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CH two, and then CH three,
another methyl group.

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So, that is some
organic shorthand.

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And I will go through that kind
of shorthand occasionally so

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that you become familiar with
it.

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And that is the hexane
molecule, a very non-polar

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molecule composed of elements,
carbon and hydrogen,

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that have very similar
electronegativity.

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And then he goes on up the
curve here, and you find this

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familiar molecule,
benzene, considered next,

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as being somehow more polar
than hexane, and liquid benzene

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accordingly in its solvating
properties in the condensed

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phase, being a more polar
medium, as it were.

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And then, as we continue
progress up this scale,

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he writes next molecules like
the diethyl ether molecule.

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No doubt you are familiar with
ether and its applications as an

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anesthetic.
The first use of ether as an

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anesthesia in an operation in
medicine was carried out just

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across the river in Boston.
There is this historical Ether

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Dome there that you can go and
look at.

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But, nonetheless,
we are just getting up a little

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bit more polar than benzene.
And then next you find

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molecules like this one.
That contains an organic

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functional group called an
ester.

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That is ethyl acetate,
which is a substance that you

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might have smelled if you ever
built models,

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because some of the glues that
you may have used would contain

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it, or the paints.
And so, it is a system that,

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in terms of its molecular
structure, is more polar.

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And, when you use it in the
condensed phase,

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it behaves in a manner that you
would describe as more polar.

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And then, finally,
you get up to some very polar

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systems, like the water
molecule.

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Like the ammonia molecule.
And, finally,

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Lewis talks about a species
like this, which is sodium twice

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sulfur.
That is actually called sodium

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sulfide.
And it is a salt analogous to

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table salt, sodium chloride.

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And being a salt,
it is obviously very polar

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because there is quite a large
degree of charge separation

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between the atoms in the
molecule.

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And, accordingly,
if you take that salt and heat

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it up until you have got it as
hot as its melting point and

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make it liquid,
that molten salt has

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individually charged ions moving
around in solution.

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And that provides a very polar
medium, which would be excellent

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at dissolving polar things.
You will notice here that on

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the non-polar side we have
elements, carbon and hydrogen,

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that are very close to each
other in electronegativity.

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And then, as we examine this
relationship,

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we start putting in elements
like oxygen or nitrogen,

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which are very electronegative
close to the upper right-hand

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side of the Periodic Table.
That introduces polarity both

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in the molecule and in
substances comprised of the

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molecule.
And then all the way over to

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the involved atoms,
sodium and sulfur are very

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different in electronegativity
and, therefore,

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have very strongly separated
charges.

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And, in thinking about this,
Lewis proposed the notion of a

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continuum.
And his musing on this are

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really quite lucid in that 
paper, but the idea was that a

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molecule, or a part of a
molecule even,

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you can analyze molecules in
terms of smaller pieces of them

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that can be identified within
the larger molecule,

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can lie on either side of a
continuum in reference to a

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number of different properties.
And the section of class that

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we are launching into today
deals with acid-base properties.

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Let's mention that first,
acid and base.

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These are two ends of a
continuum.

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But he also recognized that you
could have a molecule that would

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be an oxidant.
And the counterpart to that is

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a reductant.
And then, here is a term that

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maybe those of you who have
studied organic chemistry in

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more detail than most would
recognize.

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This is electrophile.

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Electrophile has a counterpart
called a nucleophile.

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And then, the fourth one that I
would like to mention is

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acceptor.

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And, of course,
the counterpart to that is

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donor.

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Please remember those four
pairs because when you can

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understand and make predictions
about those four properties as a

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function of molecular structure,
then you will be a chemist.

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I mean that is a large part of
what it is all about,

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understanding all the different
chemical properties that will

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make it interested in terms of
acid-base, oxidant-reductant,

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electrophile-nucleophile,
and acceptor and donor.

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And the currency of chemistry
can be seen, with reference to

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this, to be the electron.
That is what chemistry is all

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about.
And so, we will be talking a

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lot about atoms and molecules
with reference to their

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properties and how the
properties stem from knowing

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something about the electrons in
the molecule.

270
00:19:21 --> 00:19:28
And I think you are going to
find this to be quite fun.

271
00:19:28 --> 00:19:35


272
00:19:35 --> 00:19:38
With respect to the properties
of the medium,

273
00:19:38 --> 00:19:43
let's talk about a substance
like hydrogen chloride.

274
00:19:43 --> 00:19:48


275
00:19:48 --> 00:19:53
This is just to illustrate what
I was saying over there.

276
00:19:53 --> 00:19:57
If you take a molecule like
hydrogen chloride,

277
00:19:57 --> 00:20:00
and it is a gas,
of course.

278
00:20:00 --> 00:20:04
And if you make a solution of
it in hexane,

279
00:20:04 --> 00:20:10
hexane is going to be our
solvent, and then what you find

280
00:20:10 --> 00:20:16
is that in this low polarity
medium hexane solution,

281
00:20:16 --> 00:20:21
liquid hexane,
the HCl molecule remains intact

282
00:20:21 --> 00:20:27
and floats around in solution,
interacting weakly with hexane

283
00:20:27 --> 00:20:34
molecules that surround it.
But if you take this HCl

284
00:20:34 --> 00:20:40
molecule and put it into a
solvent like water,

285
00:20:40 --> 00:20:47
then something on the other end
of the continuum transpires

286
00:20:47 --> 00:20:53
because we get ionization to
produce hydrogen ions.

287
00:20:53 --> 00:20:59
And they go off and are
separated in solution from

288
00:20:59 --> 00:21:04
chloride ions.
And we will talk more about

289
00:21:04 --> 00:21:09
just what is going on when you
separate H plus from Cl

290
00:21:09 --> 00:21:13
minus and let them
diffuse apart in aqueous

291
00:21:13 --> 00:21:15
solution.
But you can see,

292
00:21:15 --> 00:21:19
I think, the dichotomy.
Either the proton shares in the

293
00:21:19 --> 00:21:24
electrons of the chloride,
or it just pops off and ionizes

294
00:21:24 --> 00:21:28
and goes out into solution.
And Lewis wanted to understand

295
00:21:28 --> 00:21:32
this.
And, to understand it,

296
00:21:32 --> 00:21:37
he developed a theory.
And his theory was the "cube

297
00:21:37 --> 00:21:42
theory." In this part we get to
draw some cubes.

298
00:21:42 --> 00:21:47
And he decided that if you
thought of the nucleus,

299
00:21:47 --> 00:21:53
which he calls the kernel,
of an atom as being located at

300
00:21:53 --> 00:21:59
the center of the system,
then if you had an atom like

301
00:21:59 --> 00:22:05
neon, which is a noble gas with
eight valance electrons,

302
00:22:05 --> 00:22:11
that these valance electrons
would want to get away from each

303
00:22:11 --> 00:22:19
other as far as possible because
they repel each other.

304
00:22:19 --> 00:22:21
They are all negatively
charged.

305
00:22:21 --> 00:22:24
This could be,
here, the neon atom.

306
00:22:24 --> 00:22:28
Here, on that cube,
is how he would represent the

307
00:22:28 --> 00:22:33
electronic structure of the neon
atom, with the eight electrons

308
00:22:33 --> 00:22:37
here represented by
peach-colored circles,

309
00:22:37 --> 00:22:42
each the vertex of a cube.
So, they got far away from each

310
00:22:42 --> 00:22:44
other.
And you can see that in the

311
00:22:44 --> 00:22:48
historical archives,
some of his original sketch

312
00:22:48 --> 00:22:52
books containing these ideas are
there, and now you will know

313
00:22:52 --> 00:22:55
what they mean.
You will see that he progressed

314
00:22:55 --> 00:23:01
beyond understanding the atom to
understanding the molecule.

315
00:23:01 --> 00:23:07
And here is one of the
molecules that he talks about.

316
00:23:07 --> 00:23:22


317
00:23:22 --> 00:23:28
Here we have a molecule
containing 14 valance electrons,

318
00:23:28 --> 00:23:34
all arranged in that manner.
And this could be,

319
00:23:34 --> 00:23:39
for example,
the I two molecule.

320
00:23:39 --> 00:23:45
And let me draw it in terms of
a dot structure,

321
00:23:45 --> 00:23:46
--

322
00:23:46 --> 00:23:52


323
00:23:52 --> 00:23:55
-- like that.
And there was known at the

324
00:23:55 --> 00:24:01
time, partly because of the
contributions of Lewis himself,

325
00:24:01 --> 00:24:05
something called the Rule of
Eight.

326
00:24:05 --> 00:24:10
And that was a reference to
this notion that most atoms want

327
00:24:10 --> 00:24:14
to have eight electrons in their
valance shell,

328
00:24:14 --> 00:24:20
and it is also something that
we call today the Octet Rule.

329
00:24:20 --> 00:24:25
So, when you see the Rule of
Eight mentioned in his paper,

330
00:24:25 --> 00:24:31
it is the familiar Octet Rule.
And you can see here that one

331
00:24:31 --> 00:24:37
of the key features is two
electrons shared equally.

332
00:24:37 --> 00:24:40
And when electrons are shared
equally like that,

333
00:24:40 --> 00:24:45
you have a molecule with
symmetrical charge distribution,

334
00:24:45 --> 00:24:48
charge referring to the
electrons where they are in

335
00:24:48 --> 00:24:51
space.
And then, he showed that you

336
00:24:51 --> 00:24:55
could think about this another
way.

337
00:24:55 --> 00:25:05


338
00:25:05 --> 00:25:10
You could consider some kind of
a geometric distortion of the I

339
00:25:10 --> 00:25:12
two molecule, like this.

340
00:25:12 --> 00:25:17
Distorted, not only because of
my drawing, but because of the

341
00:25:17 --> 00:25:22
way the atoms themselves are
thought to be rearranging in

342
00:25:22 --> 00:25:24
this process.

343
00:25:24 --> 00:25:34


344
00:25:34 --> 00:25:39
Notice that what has happened
here is that the I two

345
00:25:39 --> 00:25:44
molecule is doing something
electronically such that one

346
00:25:44 --> 00:25:49
iodine is starting to pull away
from the other.

347
00:25:49 --> 00:25:55
This one over here is carrying
with it the electron that came

348
00:25:55 --> 00:25:58
from over here.
And, in that case,

349
00:25:58 --> 00:26:04
what we have is a molecule in
which we are developing partial

350
00:26:04 --> 00:26:08
negative charge on the
right-hand side and partial

351
00:26:08 --> 00:26:14
positive charge on the left-hand
side.

352
00:26:14 --> 00:26:18
As I was talking up here,
I was thinking about what I was

353
00:26:18 --> 00:26:21
going to say here,
because the ideal is that

354
00:26:21 --> 00:26:25
molecules, even if they are
inherently non-polar like the I

355
00:26:25 --> 00:26:29
two molecule,
in their equilibrium structure

356
00:26:29 --> 00:26:33
they can have electron
fluctuations that lead to

357
00:26:33 --> 00:26:37
partial charges in the manner
shown here.

358
00:26:37 --> 00:26:39
And Lewis was working
beautifully toward the

359
00:26:39 --> 00:26:42
development of a theory like
this that would allow the

360
00:26:42 --> 00:26:45
description of electronic
structure to be made graphically

361
00:26:45 --> 00:26:48
apparent.
And this is something that

362
00:26:48 --> 00:26:51
chemists really like to do,
is we like to be able to see

363
00:26:51 --> 00:26:53
stuff.
I hope I am going to be able to

364
00:26:53 --> 00:26:56
show you some really cool stuff
this semester.

365
00:26:56 --> 00:26:58
And, in fact,
even some really cool stuff

366
00:26:58 --> 00:27:03
before we finish here today.
So, that is the development of

367
00:27:03 --> 00:27:08
charge as represented using the
Lewis cube theory in an I two

368
00:27:08 --> 00:27:12
molecule.
Next, one thing you could do is

369
00:27:12 --> 00:27:17
say, let's just continue this
process, and the right-hand

370
00:27:17 --> 00:27:20
iodine with its eight electrons
pops off.

371
00:27:20 --> 00:27:25
That would give you an ionized
situation in which you had an I

372
00:27:25 --> 00:27:32
minus floating away from
an I plus molecule.

373
00:27:32 --> 00:27:35
And that situation is drawn in
the notes, and because I am

374
00:27:35 --> 00:27:39
running out of space on this
board I am not going to draw

375
00:27:39 --> 00:27:42
that one up.
Instead I am going to go onto

376
00:27:42 --> 00:27:46
another triumph of the Lewis
cube theory, which was its

377
00:27:46 --> 00:27:50
ability to explain different
chemical bond orders.

378
00:27:50 --> 00:28:02


379
00:28:02 --> 00:28:05
By the way, Lewis never got the
Nobel Prize.

380
00:28:05 --> 00:28:08
He certainly should have.
A dramatic oversight.

381
00:28:08 --> 00:28:13
He was born in Massachusetts.
He was a Harvard professor for

382
00:28:13 --> 00:28:16
a while and did not like it,
and came to MIT to be a

383
00:28:16 --> 00:28:20
professor for a while.
And, although he liked that,

384
00:28:20 --> 00:28:23
that was not far enough away
from Harvard,

385
00:28:23 --> 00:28:27
so he went to Berkeley,
where he became the department

386
00:28:27 --> 00:28:31
head and was very successful
over a long period of time in

387
00:28:31 --> 00:28:37
learning about chemistry and in
teaching chemistry.

388
00:28:37 --> 00:28:40
I will invite you,
and you will see this in the

389
00:28:40 --> 00:28:45
notes, too, to go ahead and see
if you can find a good biography

390
00:28:45 --> 00:28:49
of Lewis because it is very
instructive.

391
00:28:49 --> 00:28:53
Anyway, here is another
molecule considered by Lewis's

392
00:28:53 --> 00:28:55
cube theory.

393
00:28:55 --> 00:29:00


394
00:29:00 --> 00:29:05
How many electrons are being
shared by the two nuclei or

395
00:29:05 --> 00:29:09
atoms involved?
Four electrons shared.

396
00:29:09 --> 00:29:15
We can circle them here.
That is four electrons shared.

397
00:29:15 --> 00:29:18
Therefore, that is not a single
bond.

398
00:29:18 --> 00:29:23
We call that a double bond.
And so, that might be a

399
00:29:23 --> 00:29:27
representation of,
for example,

400
00:29:27 --> 00:29:32
the O two molecule.
Here we have I two,

401
00:29:32 --> 00:29:37
here we have distorted I two
with charges developing,

402
00:29:37 --> 00:29:41
and here we have O two.
O two is a very

403
00:29:41 --> 00:29:45
interesting diatomic molecule.
We are going to talk about that

404
00:29:45 --> 00:29:49
later in the semester,
but for now let's just leave it

405
00:29:49 --> 00:29:53
at that.
But then, even though this was

406
00:29:53 --> 00:29:57
quite a successful theory,
the cube theory,

407
00:29:57 --> 00:30:00
it has certain problems,
certain shortcomings,

408
00:30:00 --> 00:30:04
such as, we can do single bonds
with it quite well,

409
00:30:04 --> 00:30:09
and now we can do double bonds
with the cube theory,

410
00:30:09 --> 00:30:13
but what do you do if you have
to make a triple bond?

411
00:30:13 --> 00:30:18
That was a problem for Lewis.
That should be a problem for

412
00:30:18 --> 00:30:21
you, too.
Cubes just don't fit together

413
00:30:21 --> 00:30:24
that way.
They have edges and faces,

414
00:30:24 --> 00:30:29
and that is it.
And they have vertices.

415
00:30:29 --> 00:30:33
You can do one vertex.
So, triple bond is a problem.

416
00:30:33 --> 00:30:37
And we know that there are
molecules that have triple

417
00:30:37 --> 00:30:39
bonds.
And we cannot handle them with

418
00:30:39 --> 00:30:43
the cube theory,
so we are going to have to

419
00:30:43 --> 00:30:46
something a little different,
for that reason.

420
00:30:46 --> 00:30:51
But there is also another
reason that had to do with just

421
00:30:51 --> 00:30:55
considering the chemistry of
light elements like hydrogen and

422
00:30:55 --> 00:30:58
helium.
Those elements tend to

423
00:30:58 --> 00:31:03
associate themselves with only
two electrons.

424
00:31:03 --> 00:31:08
And so, Lewis decided,
well, maybe the rule of two is

425
00:31:08 --> 00:31:14
more fundamental than the rule
of eight, and that we should be

426
00:31:14 --> 00:31:18
thinking about pairs of
electrons somehow.

427
00:31:18 --> 00:31:24
Because he looked at what was
known about chemistry and

428
00:31:24 --> 00:31:29
realized that in most molecules,
the number of electrons is

429
00:31:29 --> 00:31:35
always even, is always divisible
by two.

430
00:31:35 --> 00:31:39
That is something interesting
about two electrons at a time

431
00:31:39 --> 00:31:43
being important.
You take that together with the

432
00:31:43 --> 00:31:47
triple bond problem,
and Lewis started thinking in

433
00:31:47 --> 00:31:51
terms of electron pairs,
and this is where this

434
00:31:51 --> 00:31:55
"electron pair theory" comes
from.

435
00:31:55 --> 00:32:00


436
00:32:00 --> 00:32:04
Although he spent a lot of time
on the cube theory,

437
00:32:04 --> 00:32:09
he was perfectly willing to
cast that theory aside in favor

438
00:32:09 --> 00:32:14
of something that maybe was
going to work a little better,

439
00:32:14 --> 00:32:18
the electron pair theory.
And how does that look?

440
00:32:18 --> 00:32:23
Well, of course,
his initial pictures still had

441
00:32:23 --> 00:32:28
the cube, which just shows that
once the human mind fixes on

442
00:32:28 --> 00:32:33
something, it fixes on it with
great tenacity and it can be

443
00:32:33 --> 00:32:39
hard to shake your way of
thinking about things.

444
00:32:39 --> 00:32:42
But hopefully you will be able
to do that.

445
00:32:42 --> 00:32:45
He still decided,
let's arrange these electron

446
00:32:45 --> 00:32:50
pairs on a cube with the nucleus
of the atom supposed to be at

447
00:32:50 --> 00:32:54
the center, and let's keep these
electrons, now,

448
00:32:54 --> 00:32:58
nicely collected in pairs.
And let's get these pairs as

449
00:32:58 --> 00:33:03
far away from each other as
possible, for the same reasons

450
00:33:03 --> 00:33:08
we used to be putting eight
electrons as far away from each

451
00:33:08 --> 00:33:13
other as possible,
namely, electron repulsion.

452
00:33:13 --> 00:33:17
Now, the problem of electron
repulsion between the pairs was

453
00:33:17 --> 00:33:20
not really solved.
And Lewis decided that the pair

454
00:33:20 --> 00:33:25
theory worked so well that he
was not going to worry about

455
00:33:25 --> 00:33:27
that, and so we won't worry
about it yet,

456
00:33:27 --> 00:33:31
either.
You have your four pairs of

457
00:33:31 --> 00:33:37
electrons arranged at the center
of four of the edges of the

458
00:33:37 --> 00:33:43
cube, and that geometry is what
we call a tetrahedron,

459
00:33:43 --> 00:33:47
which I am drawing the edges of
here in pink.

460
00:33:47 --> 00:33:52
You have a tetrahedral
disposition of four pairs of

461
00:33:52 --> 00:33:55
electrons, now,
for an atom.

462
00:33:55 --> 00:34:00
This might again be the neon
atom.

463
00:34:00 --> 00:34:11
But the nice thing about this
choice is that if you want to

464
00:34:11 --> 00:34:24
you can make single bonds with
this theory, and you can make

465
00:34:24 --> 00:34:28
double bonds --

466
00:34:28 --> 00:34:33


467
00:34:33 --> 00:34:39
-- with this theory,
like that.

468
00:34:39 --> 00:34:49
And then, due to the properties
of the tetrahedron,

469
00:34:49 --> 00:35:00
you can also make triple bonds.
This is marvelous.

470
00:35:00 --> 00:35:05
You can see that the famous
chemist Gilbert Newton Lewis was

471
00:35:05 --> 00:35:09
talking about electronic
structure in terms of

472
00:35:09 --> 00:35:12
symmetrical things,
like tetrahedra and cubes,

473
00:35:12 --> 00:35:17
as a way of arranging electrons
reasonably in space.

474
00:35:17 --> 00:35:20
This could, again,
be our iodine molecule.

475
00:35:20 --> 00:35:24
This could be,
now, our O two molecule.

476
00:35:24 --> 00:35:29
And, if we continue that
progression in the Periodic

477
00:35:29 --> 00:35:35
Table, what is this molecule?
Dinitrogen, N two,

478
00:35:35 --> 00:35:41
the major component of our
atmosphere, another interesting

479
00:35:41 --> 00:35:47
molecule that we will be talking
about as the semester progresses

480
00:35:47 --> 00:35:51
forward.
But chemical language and

481
00:35:51 --> 00:35:57
electronic structure theory are
very much still permeated with

482
00:35:57 --> 00:36:03
this affection for symmetry and
the problems that you can solve

483
00:36:03 --> 00:36:10
using symmetry to analyze
electronic structure theory.

484
00:36:10 --> 00:36:15
Let's talk next about another
molecule because we want to talk

485
00:36:15 --> 00:36:20
about Lewis's development of the
electron pair theory,

486
00:36:20 --> 00:36:26
but also we want to understand
how that relates to acid-base

487
00:36:26 --> 00:36:29
properties.
And so, let me draw here

488
00:36:29 --> 00:36:32
another molecule.

489
00:36:32 --> 00:36:37


490
00:36:37 --> 00:36:41
And when you look at this
molecule, which is the aluminum

491
00:36:41 --> 00:36:45
trichloride molecule,
in which I am drawing out

492
00:36:45 --> 00:36:50
explicitly all of the electrons,
not forgetting these two.

493
00:36:50 --> 00:36:54
And having drawn the aluminum
trichloride molecule

494
00:36:54 --> 00:36:59
and looked at it like that,
you can ask yourself is the

495
00:36:59 --> 00:37:03
number of electrons divisible by
eight?

496
00:37:03 --> 00:37:06
Yes it is.
We have a 24 valance electron

497
00:37:06 --> 00:37:10
system here.
It is divisible by eight.

498
00:37:10 --> 00:37:14
Of course, it is also divisible
by two.

499
00:37:14 --> 00:37:18
You can also ask yourself is
this molecule,

500
00:37:18 --> 00:37:23
as written, one that satisfies
the rule of eight for all four

501
00:37:23 --> 00:37:25
of the atoms?
No.

502
00:37:25 --> 00:37:30
We have a problem there because
this thing is electron

503
00:37:30 --> 00:37:33
deficient.

504
00:37:33 --> 00:37:42


505
00:37:42 --> 00:37:45
And if you were Lewis,
you would want to know,

506
00:37:45 --> 00:37:50
well, if it is electron
deficient, this molecule must be

507
00:37:50 --> 00:37:54
doing something about that to
get more electrons.

508
00:37:54 --> 00:37:58
Which atom is missing
electrons, here?

509
00:37:58 --> 00:38:02
The aluminum.
How can we give aluminum more

510
00:38:02 --> 00:38:04
electrons, in a structure like
this?

511
00:38:04 --> 00:38:08
Can we get it up to the rule of
eight?

512
00:38:08 --> 00:38:13


513
00:38:13 --> 00:38:14
Double bonds,
awesome.

514
00:38:14 --> 00:38:18
Beautiful.
We can make a double bond.

515
00:38:18 --> 00:38:22
And, if we do that,
we can allow aluminum to

516
00:38:22 --> 00:38:27
satisfy the rule of eight,
or allow the rule of eight to

517
00:38:27 --> 00:38:33
satisfy aluminum.
Whichever way you want to look

518
00:38:33 --> 00:38:36
at it.
And see what we have done,

519
00:38:36 --> 00:38:39
here?
We have taken one of the

520
00:38:39 --> 00:38:42
electron pairs,
say, this one,

521
00:38:42 --> 00:38:49
and we have moved it in between
the aluminum and the chlorine,

522
00:38:49 --> 00:38:52
so it joins the other one in
there.

523
00:38:52 --> 00:38:57
And we get a double bond.
And now, everybody has an

524
00:38:57 --> 00:39:02
octet.
But what is the problem with

525
00:39:02 --> 00:39:05
that?
The problem with that is that

526
00:39:05 --> 00:39:08
chlorine is very
electronegative,

527
00:39:08 --> 00:39:13
and it does not want its
electrons stolen away by the

528
00:39:13 --> 00:39:16
very electropositive aluminum
center.

529
00:39:16 --> 00:39:21
There are other ways that
aluminum can ultimately satisfy

530
00:39:21 --> 00:39:27
its octet, and this is how we
get into the realm of "Lewis

531
00:39:27 --> 00:39:32
acid-base theory."
And I want to show you one of

532
00:39:32 --> 00:39:36
the other ways that aluminum can
satisfy its octet,

533
00:39:36 --> 00:39:41
and then, we are going to move
to the computer so we can

534
00:39:41 --> 00:39:44
visualize some of these
properties.

535
00:39:44 --> 00:39:49
If I put my aluminum here --
and do you remember this

536
00:39:49 --> 00:39:52
molecule up here,
the ammonia molecule?

537
00:39:52 --> 00:39:57
I am going to say that if I
draw that ammonia molecule in

538
00:39:57 --> 00:40:00
place, here --

539
00:40:00 --> 00:40:05


540
00:40:05 --> 00:40:10
The ammonia molecule comes in,
and it has its eight electrons.

541
00:40:10 --> 00:40:13
It is sharing six of them with
three hydrogens.

542
00:40:13 --> 00:40:18
The idea is that it has an
extra pair of electrons on that

543
00:40:18 --> 00:40:23
nitrogen in the ammonia
molecule, that it can share with

544
00:40:23 --> 00:40:26
the aluminum.
And now we go ahead and

545
00:40:26 --> 00:40:30
complete the picture.
And we can have eight electrons

546
00:40:30 --> 00:40:33
around everybody.

547
00:40:33 --> 00:40:38


548
00:40:38 --> 00:40:42
And so the rule of eight is now
satisfied for the three

549
00:40:42 --> 00:40:46
chlorines, for the aluminum,
and for the nitrogen of the

550
00:40:46 --> 00:40:50
ammonia molecule.
The ammonia molecule is what we

551
00:40:50 --> 00:40:53
call a "Lewis base."

552
00:40:53 --> 00:40:58


553
00:40:58 --> 00:41:03
And the aluminum trichloride
molecule is what we

554
00:41:03 --> 00:41:08
call a "Lewis acid,"
for reasons that should now be

555
00:41:08 --> 00:41:12
quite clear.
And I want to show you how we

556
00:41:12 --> 00:41:17
can look at these things.
And if you will give me just a

557
00:41:17 --> 00:41:22
second here, I am going to load
the aluminum trichloride

558
00:41:22 --> 00:41:26
molecule up onto the screen.

559
00:41:26 --> 00:41:33


560
00:41:33 --> 00:41:38
What we are going to do is
remember that there are lots of

561
00:41:38 --> 00:41:42
different ways in which you can
visualize molecules.

562
00:41:42 --> 00:41:48
And I want to show you some of
the more interesting ways.

563
00:41:48 --> 00:42:26


564
00:42:26 --> 00:42:33
All that is is a picture of the
aluminum trichloride

565
00:42:33 --> 00:42:40
molecule with spheres drawn at
kind of arbitrary radii.

566
00:42:40 --> 00:42:45
The size of these spheres is
not representative of any

567
00:42:45 --> 00:42:51
particular physical quantity,
but we can change that.

568
00:42:51 --> 00:42:56
And so now, what we are going
to do is draw the thing a

569
00:42:56 --> 00:43:02
slightly different way.
And what you can see is that

570
00:43:02 --> 00:43:07
the positions of those nuclei
are in this flat box.

571
00:43:07 --> 00:43:13
I made the box kind of flat.
It is kind of a slab because

572
00:43:13 --> 00:43:19
this AlCl three
molecule is conveniently kind of

573
00:43:19 --> 00:43:24
flat in its structure.
And what are we going to do

574
00:43:24 --> 00:43:28
now?
We are going to draw a contour

575
00:43:28 --> 00:43:31
surface.
And I like this.

576
00:43:31 --> 00:43:34
We are going to use the solid
surface method,

577
00:43:34 --> 00:43:37
first.
And can you see that now?

578
00:43:37 --> 00:43:41
These curves that enclose the
four nuclei of the aluminum

579
00:43:41 --> 00:43:45
trichloride molecule
do have some physical

580
00:43:45 --> 00:43:49
significance,
because what I have done here

581
00:43:49 --> 00:43:53
is taken results,
in this case from the quantum

582
00:43:53 --> 00:43:57
chemistry calculation,
but you could also take it from

583
00:43:57 --> 00:44:03
an X-ray crystallographic
structure determination study.

584
00:44:03 --> 00:44:06
And I have drawn a
three-dimensional contour map.

585
00:44:06 --> 00:44:11
The value of the electron
density at every point on the

586
00:44:11 --> 00:44:14
surface of one of those ovaloid
shapes is the same.

587
00:44:14 --> 00:44:19
Just like when you have a
contour map to describe in two

588
00:44:19 --> 00:44:22
dimensions an interesting
complicated surface,

589
00:44:22 --> 00:44:27
we can do this in three
dimensions as well.

590
00:44:27 --> 00:44:30
And one of the things you
notice, when you look at a

591
00:44:30 --> 00:44:33
surface like this,
is that the value of the

592
00:44:33 --> 00:44:37
electron density actually goes
down to some low value in

593
00:44:37 --> 00:44:39
between the nuclei,
for this.

594
00:44:39 --> 00:44:43
When you see molecules
represented as balls and sticks,

595
00:44:43 --> 00:44:47
or if you represent molecules
this way, just showing a pair of

596
00:44:47 --> 00:44:51
dots between the aluminum and
chloride, you may not be getting

597
00:44:51 --> 00:44:56
a physically complete picture of
the situation.

598
00:44:56 --> 00:44:59
You can get a much more
complete picture if you have

599
00:44:59 --> 00:45:02
access to the actual electron
density.

600
00:45:02 --> 00:45:06
And then, you can even go a
little further,

601
00:45:06 --> 00:45:09
and I want to show you
something very cool.

602
00:45:09 --> 00:45:14
First I am going to need to
load in a second molecule.

603
00:45:14 --> 00:45:19


604
00:45:19 --> 00:45:22
And that will be the product of
the first "Lewis acid-base

605
00:45:22 --> 00:45:25
reaction" that we have
discussed.

606
00:45:25 --> 00:45:29
Because, I didn't say it over
there, but that was your first

607
00:45:29 --> 00:45:33
exposure to a Lewis acid-base
reaction, in which the ammonia

608
00:45:33 --> 00:45:36
molecule comes in and binds to
the aluminum trichloride

609
00:45:36 --> 00:45:40
molecule,
in order to satisfy the valence

610
00:45:40 --> 00:45:44
requirements of the respective
atom types.

611
00:45:44 --> 00:45:47
And so, here we are going to
look at that molecule,

612
00:45:47 --> 00:45:51
and we are going to look at its
electron density.

613
00:45:51 --> 00:46:00


614
00:46:00 --> 00:46:04
And I think you are going to
like this very much.

615
00:46:04 --> 00:46:26


616
00:46:26 --> 00:46:29
Here it comes.
And then I am going to

617
00:46:29 --> 00:46:33
represent this as a solid
surface.

618
00:46:33 --> 00:46:38
The value of the electron
density everywhere on this

619
00:46:38 --> 00:46:42
surface is 0.1 electron density
units.

620
00:46:42 --> 00:46:48
I want you to start thinking
about how that shape appears

621
00:46:48 --> 00:46:53
there because I am going to
underscore that a little bit

622
00:46:53 --> 00:46:59
more in just a second.
But one thing you should notice

623
00:46:59 --> 00:47:04
is that at the NH three
molecule, which is pictured at

624
00:47:04 --> 00:47:10
the top of that representation,
what you are seeing is that the

625
00:47:10 --> 00:47:15
value of the electron density
does not get very small as you

626
00:47:15 --> 00:47:19
go between the nitrogen and
hydrogen nuclei because both

627
00:47:19 --> 00:47:25
nitrogen and hydrogen are quite
similar in electronegativity as

628
00:47:25 --> 00:47:30
compared with the aluminum atom
in this system.

629
00:47:30 --> 00:47:38


630
00:47:38 --> 00:47:46
The fascinating thing about
illustrating a molecule in this

631
00:47:46 --> 00:47:55
way is that we can actually
associate another property with

632
00:47:55 --> 00:48:02
the color on the surface.
Now what we are doing is

633
00:48:02 --> 00:48:06
looking at the aluminum
trichloride ammonia system.

634
00:48:06 --> 00:48:10
And let me see if I can make
this thing gradually spin,

635
00:48:10 --> 00:48:15
so that I can talk about it
without being pinned to my

636
00:48:15 --> 00:48:18
computer.
What we have on the surface on

637
00:48:18 --> 00:48:23
each of these electron density
isosurface enclosed regions is a

638
00:48:23 --> 00:48:28
color that corresponds with a
value of a function that tells

639
00:48:28 --> 00:48:33
us about the propensity of
electrons to be paired at that

640
00:48:33 --> 00:48:38
point in space.
The Lewis theory is now colored

641
00:48:38 --> 00:48:42
onto the electron density
isosurface of this Lewis acid,

642
00:48:42 --> 00:48:47
Lewis base complex molecule.
And the values that correspond

643
00:48:47 --> 00:48:52
to those regions in space where
electrons are most likely to be

644
00:48:52 --> 00:48:54
found paired up are colored
blue.

645
00:48:54 --> 00:48:59
Where they are least likely to
be found paired up are colored

646
00:48:59 --> 00:49:02
kind of red-orange,
here.

647
00:49:02 --> 00:49:05
And then an intermediate color
is this green.

648
00:49:05 --> 00:49:09
And so, you can see that you
have electron pairs associated

649
00:49:09 --> 00:49:12
with each of the three NH bonds,
up at the top.

650
00:49:12 --> 00:49:16
You have another electron pair
here in blue,

651
00:49:16 --> 00:49:19
which is very proximate to this
aluminum center,

652
00:49:19 --> 00:49:24
that is very electron deficient
as we had found right over here

653
00:49:24 --> 00:49:27
because it lacks an octet.
And then, each of these

654
00:49:27 --> 00:49:32
chlorines is able to attract
electron pairs.

655
00:49:32 --> 00:49:34
Because chlorine is very
electronegative,

656
00:49:34 --> 00:49:39
it attracts the electron pairs
to itself and sort of withholds

657
00:49:39 --> 00:49:43
them from the aluminum center,
but yet, that development of

658
00:49:43 --> 00:49:47
charge separation keeps
everything together in what we

659
00:49:47 --> 00:49:51
can call an ionic interaction,
an electrostatic interaction.

660
00:49:51 --> 00:49:55
You can see here covalent
bonding, donor-acceptor complex

661
00:49:55 --> 00:50:00
formation, and electrostatic
contributions to bonding.

662
0:50:00 --> 00:50:03
And all of this is just an
illustration of the brilliant

663
0:50:03.942 --> 50:06
ideas of Gilbert Newton Lewis.