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What I am going to do today is I am going to start talking
about the development of atomic theory.
I am going to whiz through what the evidence is for the
existence of atoms. And then, we are going to talk
about how the atom is not the most basic constituent of
matter, how the atom can be divided into at least an
electron and a nucleus. And then, what we are going to
see is how the existing classical way of thinking,
Newtonian mechanics, cannot explain how that
electron and that nucleus hangs together.
And later on in the course, we are going to see how that
existing classical physics is not going to be able to explain
how two atoms hang together. We are going to look at the
fundamental principles, here, of chemical bonding.
I am going to get going on this subject.
Then, about three-quarters of the way through,
I am going to stop. And then I will do some
introductions of our teaching team this semester.
And then also we will talk about the mechanics of the
course and some expectations of the course.
Let's get going. Certainly, the Ancient Greeks
were known to have pondered whether matter can be divided ad
infinitum into smaller and smaller pieces,
chopped up into smaller and smaller pieces,
or whether there was a point at which you couldn't chop up
matter any further. Aristotle over here was one of
those philosophers who believed that matter was infinitely
divisible. You could chop it up ad
infinitum. This is called the continuum
theory of matter. It is a continuum.
There is no discreteness to matter.
That was his view of the structure of matter,
but there was a minority opinion.
An opinion actually held by Democritus who was 100 years
older than Aristotle. And Democritus believed that
matter was composed of discrete particles called,
in Greek, "atomos," "a" meaning not, "tomos" meaning divisible,
not divisible particles. Well, for whatever reason,
Aristotle's continuum theory of matter prevailed all the way up
to the 17th century. And here he is depicted by
Raphael, the frescos on the walls in the Vatican holding
court on the continuum theory of matter.
But at the same time that Raphael actually painted this
picture, there were beginning to accumulate some observations
about how matter behaved and how it reacted that did not quite
jive with this continuum theory of matter.
And what were those observations?
Well, one of those observations was by this gentleman,
Robert Boyle. Guess what his profession was.
Chemist? Good guess.
He was actually a theologian, as most chemists were at that
time. You know him largely for the
empirical observation that if you take the pressure times the
volume of a gas, it is always a constant.
At least when the temperature is constant.
But Robert Boyle also put forth probably the first idea of an
element. And he called elements certain
primitive unmingled bodies. And he also put forth the idea
that these unmingled bodies were the ingredients of perfectly
mixed bodies. Just a pseudonym for molecules,
for compounds. And then there is the work of
this gentleman, Joseph Priestley.
Guess what his occupation was. Right, he was a priest.
And what he did was he carried out some reactions of
dephlogisticated air with various materials.
And what he found was that materials reacted more
vigorously in dephlogisticated air than they did in
undephlogisticated air. And, of course,
dephlogisticated air is nothing other than oxygen.
It is the air with the nitrogen removed from it.
But it really took this gentleman, Lavoisier,
to understand what Priestley's experiments were all about.
And what Lavoisier realized is that when materials were
reacting with this dephlogisticated air,
this dephlogisticated air was kind of adding to the material.
And he came to that conclusion because he did some very careful
measurements of the mass of the dephlogisticated air plus the
material before the reaction and some careful measurements after.
And found that they were indeed equal.
There was a conservation of mass.
And from that Lavoisier was really the first person to
realize that a chemical reaction was analogous to an algebraic
equation. He also went on to isolate 17
different metals and identified them as elements and nine
different nonmetals and identified them as elements.
But for all of his efforts, well, we all know what happened
to him. He was advisor to the French
Monarchy. The judge at his trial
proclaimed the Republic has no use for Savants.
LaGrange, who was a mathematician at that time,
said, "It took but a moment to cut off that head,
though 100 years will be required to produce another like
it." Well, here we have some observations and we have some
observations -- Oh, I forgot one other person
here. That's this guy,
J. L.
Proust. J.
L. Proust was also a French
scientist at that time, but he was a little more
politically savvy. And so he high-tailed it out of
France and lived a long and productive life as a professor
in Madrid. And what he did were
experiments. He recognized from the results
that when two elements combine to form a given compound,
they always did so in definite proportions by weight,
regardless of what kind of method of preparation he used to
make that particular compound. Here is an example where matter
didn't quite behave as a continuum.
There was a discreteness of some sense to matter.
And it really took John Dalton, an English schoolteacher with
broad interests, to realize, or to recognize
these observations of Priestley, of Lavoisier,
of Proust that he could understand all of these
observations if he resurrected the idea of Democritus,
the idea of atomos, or atoms.
And so he forth some postulates.
Well, those postulates are now known as Dalton's Atomic Theory,
but they were postulates at the time.
And those postulates say each element is composed of atoms,
atoms of a given element are identical and that compounds
form when atoms of more than one element combine.
And, of course, that atoms are not created or
destroyed. And then, just an aside,
Dalton, with his very broad range of interest,
was also really the first person to document
colorblindness in humans. Colorblindness is also called
Daltonism. You see, we are getting you set
for medical school already. But I want you to recognize
here that Dalton didn't actually do any of these experiments
himself. I think he could have,
but he didn't. Instead he just said that if
Lavoisier was right and Proust's observations are right,
well, then I can understand those observations in terms of
this framework of postulates. And I point this out because
this is a powerful method in science, a powerful way in which
science works in that there are often some observations
seemingly disparate. And then somebody comes along
and recognizes a unifying factor.
In this case, the presence of atoms or
discrete particles. Now, of course,
Dalton's Atomic Theory here was not immediately accepted.
And rightfully so. It needed further
substantiation. And that further substantiation
came in the form of work by this gentleman, Joseph Gay-Lussac,
the Law of Combining Volumes. It came in the form of work by
Lorenzo Romano Amedeo Carlo Avogadro's hypothesis.
And here I want you to realize that, even though you didn't
know it, you indeed can read Italian.
It says "equal volumes of gases under the same conditions of
temperature yield the same number of molecules or atoms."
There you go. That is from an Italian stamp.
You can read Italian. And that further substantiation
came from the work of this gentleman, Ludwig Boltzmann,
gas kinetic theory, who recognized - you know - the
pressure of a gas that must be due to individual particles that
are moving and that are ramming into the walls of some vessel.
That must be what gives rise to pressure.
And then, finally, it took a statesman,
Cannizzaro. And what Cannizzaro did was he
got the scientific establishment at that time,
and the scientific establishment at that time for
sure was a small group of pale males, to listen to Dalton's
Atomic Theory and to the supporting data from Avogadro
and company. And ultimately got them to say,
yes, there is something to it here.
And so by the late 1800s, the idea of atoms was pretty
strongly ingrained in the scientific community.
Now, of course nowadays we can actually see individual atoms
for molecules. And so here is a picture of 28
individual CO molecules arranged in the form of a little man or a
little woman, I don't know which.
And each one of these CO molecules is an orange ball.
And what you are looking at are these CO molecules bound to a
platinum surface. They are bound to a platinum
surface such that the carbon end is down and the oxygen end is
up. And there is a really good
reason why the carbon end is down and the oxygen end is up.
And Professor Cummins, whom I am going to introduce to
you in a few minutes, is going to talk to you about
what that really good reason is in the second half of the
course. We know why this is case.
What you are looking at here is really looking at the oxygen end
of the CO molecule, because we are looking at a top
view. All right.
How is this image made? Well, this image was made by a
technique called scanning tunneling microscopy that was
invented before you were born, I am sorry to say.
I am sorry for myself to say. It was worked on by Ruska and
then perfected by Binnig and Rohrer.
And they earned themselves a Nobel Prize for this work.
And the way this techniques works is the following.
What you are going to do is take a thin tungsten wire.
It might be 0.01 inches in diameter.
And you etch it down to a fine tip.
You stick it in some potassium hydroxide, do a little
electrochemistry and etch it down to as fine a tip as you can
make it. Then you attach that tungsten
wire to something called a piezoelectric crystal.
And a piezoelectric material is one in which,
if you put a voltage across it, you can make it expand a little
bit, 10, 20 angstroms or so. And if you can make it expand a
little bit like that, well, then you've got control
on an angstrom-type level. You attach it to some
piezoelectric crystal here then it allows you to move that
tungsten tip by a very, very small amount.
You bring that tungsten tip close to the top of this CO
molecule sitting on this platinum surface.
And say you bring it to within, oh, I don't know,
5 angstroms or so from the oxygen atom.
Now, the tungsten has electrons.
And since this is a bulk metal, some of those electrons are not
firmly attached to a particular nuclei.
There is a sea of electrons. And what we are going to do is
put a negative potential on that tungsten tip.
And we are going to ground here this platinum surface.
Now, these electrons on the tungsten, they are in this
environment of a negative potential.
And that is a high-energy state for them because they are
negatively charged particles. If I were to draw here an
energy level diagram, I am going to represent then
the energy level of the electrons here in this tungsten,
around this tungsten tip as some high energy over here.
There are electrons on the tungsten.
Whereas, the electrons associated with the platinum and
the CO here, all of which are in contact with each other,
well, they are at ground. That is a lower energy state
for these negatively charged particles.
We are going to represent it by this.
This is the electrons on platinum.
And so this axis here is kind of a distance access.
These are the electrons on the tip.
These are the electrons on the platinum.
We are measuring kind of distance here,
from here to here, in the vertical direction
there. There is a thermodynamic
driving force for the electrons on the tungsten tip to want to
be here on the platinum, but the problem is that this
tungsten is not in contact with this oxygen end here.
There is a gap. And if this is in a vacuum,
we call this a vacuum gap. And so for an electron to be
inside of this vacuum gap, well, that is a very high
energy state for those electrons.
And so, if we were to look at an energy level diagram here,
that energy actually goes up pretty high before it comes back
down. There is a barrier to getting
the electrons from the tip to the platinum surface.
Well, you have seen this kind of reaction coordinate before.
You have, I am sure. If you look at an energy level
diagram here for some reactions, sometimes you have reactants
here at a high energy and products here at a lower energy.
And, in order to get from reactants to products,
there is a barrier, an activation energy barrier.
You called it E(act) or something like that.
And you know that in chemical reactions typically what you
have got to do is put energy into the system in order to get
over this barrier before you get any energy out,
before the reaction can proceed.
And that is what happens in a lot of chemical reactions.
There is a barrier and you have got to supply that energy to get
over it before you can make the reaction go.
But over here, in the case of electrons,
those electrons don't act like atoms and molecules do,
for the most part. These electrons,
what do they do? They ignore this barrier and
tunnel right through the barrier, go right through that
brick wall. How can they do that?
Well, they can do that because they are quantum mechanical
particles. We cannot treat those electrons
like we treat atoms and molecules which,
for the most part, behave as classical particles.
And, actually, this is going to be the subject
of the first few lectures here, the need for this new kind of
mechanics to explain phenomenon like this and to explain
chemical bonding. All right.
So, these electrons tunnel right through.
What does that mean? Well, what that means here,
for this experiment, is that if I then take and
attach a wire to the tungsten tip and a wire to the platinum
surface and I put an ammeter in between, I will see a
measurement of a current. There are electrons going from
this tungsten tip to this platinum surface.
I measure a current. Well, that is nice.
But now, from that, how do I get this image of
these 28 CO molecules? Well, what I do is I also have
this tungsten tip mounted not only on a piezoelectric crystal
that allows me to go up and down.
But another piezoelectric crystal, which allows me to move
it from side to side with control on the order of an
angstrom. I can take the tungsten tip and
I am going to move it over by a certain amount.
And exactly I am going to know that certain amount is because I
calibrated my piezoelectric crystal.
But now, when I move that tungsten tip over,
what is going to happen to this current here?
Pardon? It is going to go down.
It is going to plummet. It is going to go to zero.
Why? Because when I move this tip
over, I increase the distance between the end of the tip and
the oxygen onto the molecule. And when I increase the
distance, what I do is make this barrier wider.
And the wider the barrier is, the more difficult it is for
those electrons to tunnel through.
And so the current actually goes down.
To compensate for that, I am now going to take the
tungsten tip and move it down by just enough such that I
reestablish the current that I originally have.
And, of course, I know exactly by how much I
moved it down because, again, I have my tip
calibrated. I've got two points now.
I need a third point. I am going to take that
tungsten tip and I am going to move it over again.
Again, the current is going to go down.
But, in order to reestablish the original current I had,
I am going to move this down further.
And, again, I will know how much I move the tip over and I
will know exactly how much I move the tip down.
To get that image, I am going to provide a color
code. I am going to say that when the
tungsten tip is the largest distance away from the surface,
well, then that is going to show up here on this picture and
it is going to show up as a very light color.
That is the highest points. When the tungsten tip is a
little bit closer to the surface, well,
that is going to show up as the darker colors.
And, as it gets lower and lower and lower, it is going to be
deeper and deeper orangey. Finally, when I am actually
tunneling to the platinum surface, instead of a CO
molecule, I am going to make that a blue color.
That is how we get the image of this molecular man.
Now, you do see that these CO molecules are in the form of a
little person. This does not represent,
in the most au courant language, intelligent design.
Rather, this represents the work of a very patient
experimentalist, i.e., graduate student,
who spent 24 hours, and I know this to be the case,
moving these CO molecules into this particular arrangement.
How did he do that? Well, how he did that is the
following. What he did was took this
platinum surface and then opened up a bottle of CO in the vacuum
chamber, or let some CO into the vacuum chamber,
and the molecules just absorbed anywhere they wanted to,
well, sort of anywhere they wanted to on this platinum
surface. First of all,
he had to figure out where the molecules were.
The soccer balls are the CO molecules and the tungsten tip
here is my leg and my foot. And so the first thing he did
was scan the surface in order to figure out where the CO
molecules are and goes, okay, I know where they are.
Then he brought this tip down right next to one of the CO
molecules. Then he gave that piezoelectric
crystal a pulse of voltage which jerked it and the CO molecule
went flying away. Well, that is nice,
but now where is it? Again, you have got to go scan
along the whole surface to find it.
Well, he pushed it over too far.
Now we have got to come over here, put the tip down,
give another voltage pulse before his tip breaks.
Well, you get the idea. 24 hours later,
you've got it. This is the beginnings of
nanotechnology. You can see that it is going to
be a long time before manipulation of individual atoms
and molecules like this, one at a time,
before that competes effectively with synthesis in a
beaker where you get the molecules right where you want
them because of chemistry instead of this mechanical
manipulation. Well, even though 100 years ago
these direct observations of atoms and molecules was not
possible, it was by the late 1800s pretty well accepted,
or the evidence for the atomic structure of matter,
atoms as the most basic constituent of matter,
that evidence was really pretty compelling.
And, in fact, by the late 1800s,
it was basically believed that the theoretical structure of the
universe was complete. Nature was understood.
There were no big discoveries to be made yet.
And, in fact, there was some justification
for that attitude, because certainly by the late
1800s Newtonian mechanics, the mechanics that described
how bodies all around us moved, including astronomical bodies,
well, that had already been known for over 200 years.
Thermodynamics was formulated already by that time.
Statistical mechanics was also formulated by that time.
Statistical mechanics is a field that relates the
microscopic description of matter to the macroscopic
behavior of matter. And, very importantly,
there were experiments by Young, Fresnel and Hertz that
seemed to put to rest the notion that light was a particle.
Those experiments really nailed, or seemed to nail the
idea of light as a wave, light has wavelike particles.
They verified Maxwell's equations that unified the
fields of optics and electromagnetism.
All of these accomplishments surely did justify a very proud
feeling amongst the scientific community.
And, at that time, the feeling was that the work
that remained was largely to investigate the next decimal
place and that is it. Well, if you look really
carefully, though, at the evidence,
even with all of these accomplishments,
there were beginning to be, in the late 1800s,
some experiments that were suggesting that,
one, maybe the atom was not the most basic constituent of
matter. That was the first set of
measurements that indicated something was amiss,
the fact that the atom wasn't the most elementary particle.
And we are going to look at these sets of measurements.
Second, the other observation that hinted that this classical
thinking as amiss was the observation of the photoelectric
effect. Because the photoelectric
effect, what it did was it showed that light was behaving
like a particle and not a wave, and that sent a lot of
consternation throughout the scientific community.
We are going to look at these two tracks.
And we are going to start by talking about the fact that the
atom is not the most basic constituent of matter.
That at least you can divide the atom up into an electron and
a nucleus. We are going to start here with
this gentleman, J.J.
Thompson. Remember that name.
It is going to come back. Discovery of the electron.
This is 1897. What J.J.
Thompson was interested in doing was understanding what a
discharge was, or what made up a discharge.
For example, if you have a glass vessel that
you evacuate and then you have a cathode in that glass vessel and
you have an anode in that glass vessel, and you also put some
molecular hydrogen in it, fill it up with molecular
hydrogen. But now what you do is put a
negative voltage on the cathode and a positive voltage on the
anode. And you crank up the potential
energy difference, the voltage difference between
the cathode and the anode. And you keep cranking it up.
You have to get really pretty high, but at some point all of a
sudden what happens is that the gas here begins to blow.
And you get the establishment of this discharge,
this plasma. And J.J.
Thompson was just interested in finding out what was in this
plasma. What he did to investigate it
is he punched a hole in this anode right here and let out a
little bit of this plasma. He let it impinge on a kind of
phosphor screen here. Even though the plasma leaking
out was kind of glowing in the dark, well, it also was glowing
when it hit the phosphor screen. That lit up.
But then he took a pair of parallel metal plates above and
below this luminous beam and put a potential difference on them,
some delta V. And this delta V is just a
fraction of what this delta V is, so it is very small.
But what he noticed is that some of this luminous beam was
actually attracted toward that positively charged plate.
And so, if you have got something that is attracted to
this positively charged plate, what does it mean about this
particle? It is negatively charged.
It is just Coulomb's interaction.
And he could measure right here the amount of deflection from
the center line. I am going to call that amount
of deflection delta X sub minus to indicate that
this looks like a negatively charged particle.
Now, Thompson also knew enough electromagnetism at that time to
realize that the amount of that deflection has to be directly
proportional to the charge on that particle.
In other words, the greater the charge the
larger the deflection. I am going to represent that
charge by E sub minus. He also recognized that the
heavier that particle, the more difficult it is going
to be to deflect the particle to the positively charged plate.
That amount of deflection is going to be inversely
proportional to the mass of that negatively charged particle.
But then Thompson did a further experiment.
He increased delta V even more. And here, I am taking a little
liberty with the story. It is a little bit more
complicated, but I am just trying to get the essence here
across. He cranked this up some more.
And then, if you looked really, really carefully,
what happened is he also saw some of this being deflected
toward the negatively charged plate.
Indicating that, lo and behold,
there must also be some positively charged particles in
this luminous beam. And he called that deflection
delta X sub plus. Again, the amount of deflection
for the positively charged particles has to be proportional
to the charge on that positively charged particle and inversely
proportional to the mass of that positively charged particle.
But the other critical observation that he made was
that the amount of deflection for a given voltage,
for that negatively charged particle was much,
much larger that the amount of deflection for the positively
charged particle. That is the evidence.
Now we have to think. Now we have to make some
guesses. What he guessed is that the
positively charged particles here were H plus.
How did he know that? Well, what he did know is that
in this plasma there were some neutral hydrogen atoms.
He knew that. How he knew that I am going to
tell you, or we are going to talk about in a few days.
But he knew that this plasma takes the H two molecule
and tears it apart and makes hydrogen atoms.
And he knew it was neutral. And so he reasoned that what
must be happening is that something has to be coming off
of this hydrogen atom to make it a positively charged particle.
He said, okay, this is going to be H plus.
But then, because this was
neutral to begin with, whatever came off of the
hydrogen has to be that negatively charged particle so
that when they come together they are neutral,
because a hydrogen atom is neutral.
And, of course, ultimately that negatively
charged particle was called an electron.
But the key point is that he said, well, it must be then,
when these two particles come together, you are going to have
a neutral particle. It must be that the absolute
magnitude of the charge on that negatively charged particle,
the electron, that has to equal the magnitude
of the charge on that positively charged particle,
this hydrogen plus ion. That was the conjecture.
Well, if that is the case, if I then take a ratio of delta
X minus the delta X plus, what I am going to get here is
just that it will be equal to the mass of the hydrogen ion
divided by the mass of this negatively charged particle,
the electron. But the observation here is
that this deflection, for the negatively charged
particle, is much larger than it was for the positively charged
particle. That must mean,
if this is an equality, that the mass of this hydrogen
ion is much, much greater than the mass of this negatively
charged particle, the electron.
And this is the stunning result.
It is stunning because, at the time,
it was already known that a hydrogen atom is the least
massive atom. There wasn't evidence for any
other atom less massive than a hydrogen atom.
And so here, in this experiment,
what we are finding is that there is a particle that is less
massive than the hydrogen atom. You can chop the hydrogen atom
up. The atom is not the most basic
constituent of matter. There is some other particle
here, which we are going to call an electron, that is less
massive than a hydrogen atom. That was the first piece of
evidence for being able to split that hydrogen atom,
or that atom up into smaller particles.
The first evidence that the atom was not the most basic
constituent of matter. Now, it took another ten years,
the Millikan oil-drop experiment, for this ratio to
actually be measured accurately and for the mass of the electron
to actually be measured accurately.
And what we now know is that it takes 1,836 masses of the
electron to equal the mass of a hydrogen atom.