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What I am going to do today is
I am going to start talking

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about the development of atomic
theory.

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I am going to whiz through what
the evidence is for the

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existence of atoms.
And then, we are going to talk

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about how the atom is not the
most basic constituent of

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matter, how the atom can be
divided into at least an

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electron and a nucleus.
And then, what we are going to

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see is how the existing
classical way of thinking,

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Newtonian mechanics,
cannot explain how that

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electron and that nucleus hangs
together.

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And later on in the course,
we are going to see how that

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existing classical physics is
not going to be able to explain

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how two atoms hang together.
We are going to look at the

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fundamental principles,
here, of chemical bonding.

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I am going to get going on this
subject.

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Then, about three-quarters of
the way through,

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I am going to stop.
And then I will do some

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introductions of our teaching
team this semester.

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And then also we will talk
about the mechanics of the

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course and some expectations of
the course.

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Let's get going.
Certainly, the Ancient Greeks

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were known to have pondered
whether matter can be divided ad

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infinitum into smaller and
smaller pieces,

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chopped up into smaller and
smaller pieces,

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or whether there was a point at
which you couldn't chop up

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matter any further.
Aristotle over here was one of

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those philosophers who believed
that matter was infinitely

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divisible.
You could chop it up ad

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infinitum.
This is called the continuum

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theory of matter.
It is a continuum.

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There is no discreteness to
matter.

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That was his view of the
structure of matter,

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but there was a minority
opinion.

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An opinion actually held by
Democritus who was 100 years

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older than Aristotle.
And Democritus believed that

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matter was composed of discrete
particles called,

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in Greek, "atomos," "a" meaning
not, "tomos" meaning divisible,

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not divisible particles.
Well, for whatever reason,

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Aristotle's continuum theory of
matter prevailed all the way up

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to the 17th century.
And here he is depicted by

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Raphael, the frescos on the
walls in the Vatican holding

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court on the continuum theory of
matter.

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But at the same time that
Raphael actually painted this

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picture, there were beginning to
accumulate some observations

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about how matter behaved and how
it reacted that did not quite

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jive with this continuum theory
of matter.

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And what were those
observations?

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Well, one of those observations
was by this gentleman,

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Robert Boyle.
Guess what his profession was.

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Chemist?
Good guess.

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He was actually a theologian,
as most chemists were at that

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time.
You know him largely for the

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empirical observation that if
you take the pressure times the

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volume of a gas,
it is always a constant.

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At least when the temperature
is constant.

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But Robert Boyle also put forth
probably the first idea of an

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element.
And he called elements certain

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primitive unmingled bodies.
And he also put forth the idea

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that these unmingled bodies were
the ingredients of perfectly

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mixed bodies.
Just a pseudonym for molecules,

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for compounds.
And then there is the work of

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this gentleman,
Joseph Priestley.

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Guess what his occupation was.
Right, he was a priest.

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And what he did was he carried
out some reactions of

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dephlogisticated air with
various materials.

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And what he found was that
materials reacted more

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vigorously in dephlogisticated
air than they did in

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undephlogisticated air.
And, of course,

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dephlogisticated air is nothing
other than oxygen.

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It is the air with the nitrogen
removed from it.

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But it really took this
gentleman, Lavoisier,

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to understand what Priestley's
experiments were all about.

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And what Lavoisier realized is
that when materials were

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reacting with this
dephlogisticated air,

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this dephlogisticated air was
kind of adding to the material.

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And he came to that conclusion
because he did some very careful

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measurements of the mass of the
dephlogisticated air plus the

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material before the reaction and
some careful measurements after.

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And found that they were indeed
equal.

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There was a conservation of
mass.

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And from that Lavoisier was
really the first person to

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realize that a chemical reaction
was analogous to an algebraic

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equation.
He also went on to isolate 17

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different metals and identified
them as elements and nine

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different nonmetals and
identified them as elements.

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But for all of his efforts,
well, we all know what happened

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to him.
He was advisor to the French

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Monarchy.
The judge at his trial

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proclaimed the Republic has no
use for Savants.

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LaGrange, who was a
mathematician at that time,

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said, "It took but a moment to
cut off that head,

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though 100 years will be
required to produce another like

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it." Well, here we have some
observations and we have some

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observations --
Oh, I forgot one other person

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here.
That's this guy,

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J.
L.

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Proust.
J.

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L.
Proust was also a French

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scientist at that time,
but he was a little more

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politically savvy.
And so he high-tailed it out of

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France and lived a long and
productive life as a professor

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in Madrid.
And what he did were

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experiments.
He recognized from the results

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that when two elements combine
to form a given compound,

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they always did so in definite
proportions by weight,

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regardless of what kind of
method of preparation he used to

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make that particular compound.
Here is an example where matter

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didn't quite behave as a
continuum.

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There was a discreteness of
some sense to matter.

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And it really took John Dalton,
an English schoolteacher with

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broad interests,
to realize, or to recognize

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these observations of Priestley,
of Lavoisier,

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of Proust that he could
understand all of these

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observations if he resurrected
the idea of Democritus,

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the idea of atomos,
or atoms.

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And so he forth some
postulates.

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Well, those postulates are now
known as Dalton's Atomic Theory,

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but they were postulates at the
time.

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And those postulates say each
element is composed of atoms,

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atoms of a given element are
identical and that compounds

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form when atoms of more than one
element combine.

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And, of course,
that atoms are not created or

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destroyed.
And then, just an aside,

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Dalton, with his very broad
range of interest,

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was also really the first
person to document

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colorblindness in humans.
Colorblindness is also called

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Daltonism.
You see, we are getting you set

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for medical school already.
But I want you to recognize

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here that Dalton didn't actually
do any of these experiments

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himself.
I think he could have,

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but he didn't.
Instead he just said that if

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Lavoisier was right and Proust's
observations are right,

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well, then I can understand
those observations in terms of

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this framework of postulates.
And I point this out because

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this is a powerful method in
science, a powerful way in which

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science works in that there are
often some observations

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seemingly disparate.
And then somebody comes along

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and recognizes a unifying
factor.

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In this case,
the presence of atoms or

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discrete particles.
Now, of course,

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Dalton's Atomic Theory here was
not immediately accepted.

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And rightfully so.
It needed further

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substantiation.
And that further substantiation

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came in the form of work by this
gentleman, Joseph Gay-Lussac,

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the Law of Combining Volumes.
It came in the form of work by

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Lorenzo Romano Amedeo Carlo
Avogadro's hypothesis.

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And here I want you to realize
that, even though you didn't

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know it, you indeed can read
Italian.

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It says "equal volumes of gases
under the same conditions of

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temperature yield the same
number of molecules or atoms."

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There you go.
That is from an Italian stamp.

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You can read Italian.
And that further substantiation

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came from the work of this
gentleman, Ludwig Boltzmann,

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gas kinetic theory,
who recognized - you know - the

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pressure of a gas that must be
due to individual particles that

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are moving and that are ramming
into the walls of some vessel.

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That must be what gives rise to
pressure.

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And then, finally,
it took a statesman,

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Cannizzaro.
And what Cannizzaro did was he

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got the scientific establishment
at that time,

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and the scientific
establishment at that time for

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sure was a small group of pale
males, to listen to Dalton's

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Atomic Theory and to the
supporting data from Avogadro

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and company.
And ultimately got them to say,

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yes, there is something to it
here.

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And so by the late 1800s,
the idea of atoms was pretty

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strongly ingrained in the
scientific community.

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Now, of course nowadays we can
actually see individual atoms

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for molecules.
And so here is a picture of 28

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individual CO molecules arranged
in the form of a little man or a

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little woman,
I don't know which.

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And each one of these CO
molecules is an orange ball.

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And what you are looking at are
these CO molecules bound to a

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platinum surface.
They are bound to a platinum

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surface such that the carbon end
is down and the oxygen end is

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up.
And there is a really good

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reason why the carbon end is
down and the oxygen end is up.

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And Professor Cummins,
whom I am going to introduce to

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you in a few minutes,
is going to talk to you about

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what that really good reason is
in the second half of the

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course.
We know why this is case.

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What you are looking at here is
really looking at the oxygen end

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of the CO molecule,
because we are looking at a top

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view.
All right.

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How is this image made?
Well, this image was made by a

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technique called scanning
tunneling microscopy that was

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invented before you were born,
I am sorry to say.

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I am sorry for myself to say.
It was worked on by Ruska and

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then perfected by Binnig and
Rohrer.

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And they earned themselves a
Nobel Prize for this work.

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And the way this techniques
works is the following.

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What you are going to do is
take a thin tungsten wire.

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It might be 0.01 inches in
diameter.

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And you etch it down to a fine
tip.

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You stick it in some potassium
hydroxide, do a little

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electrochemistry and etch it
down to as fine a tip as you can

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make it.
Then you attach that tungsten

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wire to something called a
piezoelectric crystal.

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And a piezoelectric material is
one in which,

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if you put a voltage across it,
you can make it expand a little

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bit, 10, 20 angstroms or so.
And if you can make it expand a

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little bit like that,
well, then you've got control

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on an angstrom-type level.
You attach it to some

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piezoelectric crystal here then
it allows you to move that

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tungsten tip by a very,
very small amount.

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You bring that tungsten tip
close to the top of this CO

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molecule sitting on this
platinum surface.

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And say you bring it to within,
oh, I don't know,

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5 angstroms or so from the
oxygen atom.

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Now, the tungsten has
electrons.

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And since this is a bulk metal,
some of those electrons are not

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firmly attached to a particular
nuclei.

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00:15:45 --> 00:15:51
There is a sea of electrons.
And what we are going to do is

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00:15:51 --> 00:15:58
put a negative potential on that
tungsten tip.

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00:15:58 --> 00:16:02
And we are going to ground here
this platinum surface.

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00:16:02 --> 00:16:06
Now, these electrons on the
tungsten, they are in this

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00:16:06 --> 00:16:09
environment of a negative
potential.

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00:16:09 --> 00:16:13
And that is a high-energy state
for them because they are

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negatively charged particles.
If I were to draw here an

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00:16:18 --> 00:16:22
energy level diagram,
I am going to represent then

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the energy level of the
electrons here in this tungsten,

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00:16:26 --> 00:16:32
around this tungsten tip as
some high energy over here.

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There are electrons on the
tungsten.

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Whereas, the electrons
associated with the platinum and

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the CO here, all of which are in
contact with each other,

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00:16:42 --> 00:16:46
well, they are at ground.
That is a lower energy state

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00:16:46 --> 00:16:49
for these negatively charged
particles.

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We are going to represent it by
this.

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This is the electrons on
platinum.

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00:16:55 --> 00:17:00
And so this axis here is kind
of a distance access.

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00:17:00 --> 00:17:03
These are the electrons on the
tip.

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00:17:03 --> 00:17:06
These are the electrons on the
platinum.

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00:17:06 --> 00:17:10
We are measuring kind of
distance here,

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00:17:10 --> 00:17:13
from here to here,
in the vertical direction

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00:17:13 --> 00:17:16
there.
There is a thermodynamic

240
00:17:16 --> 00:17:22
driving force for the electrons
on the tungsten tip to want to

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00:17:22 --> 00:17:26
be here on the platinum,
but the problem is that this

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00:17:26 --> 00:17:33
tungsten is not in contact with
this oxygen end here.

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00:17:33 --> 00:17:36
There is a gap.
And if this is in a vacuum,

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00:17:36 --> 00:17:41
we call this a vacuum gap.
And so for an electron to be

245
00:17:41 --> 00:17:45
inside of this vacuum gap,
well, that is a very high

246
00:17:45 --> 00:17:48
energy state for those
electrons.

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00:17:48 --> 00:17:53
And so, if we were to look at
an energy level diagram here,

248
00:17:53 --> 00:17:58
that energy actually goes up
pretty high before it comes back

249
00:17:58 --> 00:18:02
down.
There is a barrier to getting

250
00:18:02 --> 00:18:07
the electrons from the tip to
the platinum surface.

251
00:18:07 --> 00:18:12
Well, you have seen this kind
of reaction coordinate before.

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00:18:12 --> 00:18:16
You have, I am sure.
If you look at an energy level

253
00:18:16 --> 00:18:22
diagram here for some reactions,
sometimes you have reactants

254
00:18:22 --> 00:18:28
here at a high energy and
products here at a lower energy.

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00:18:28 --> 00:18:31
And, in order to get from
reactants to products,

256
00:18:31 --> 00:18:35
there is a barrier,
an activation energy barrier.

257
00:18:35 --> 00:18:38
You called it E(act) or
something like that.

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00:18:38 --> 00:18:43
And you know that in chemical
reactions typically what you

259
00:18:43 --> 00:18:47
have got to do is put energy
into the system in order to get

260
00:18:47 --> 00:18:51
over this barrier before you get
any energy out,

261
00:18:51 --> 00:18:53
before the reaction can
proceed.

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00:18:53 --> 00:18:59
And that is what happens in a
lot of chemical reactions.

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00:18:59 --> 00:19:04
There is a barrier and you have
got to supply that energy to get

264
00:19:04 --> 00:19:08
over it before you can make the
reaction go.

265
00:19:08 --> 00:19:12
But over here,
in the case of electrons,

266
00:19:12 --> 00:19:16
those electrons don't act like
atoms and molecules do,

267
00:19:16 --> 00:19:19
for the most part.
These electrons,

268
00:19:19 --> 00:19:23
what do they do?
They ignore this barrier and

269
00:19:23 --> 00:19:28
tunnel right through the
barrier, go right through that

270
00:19:28 --> 00:19:32
brick wall.
How can they do that?

271
00:19:32 --> 00:19:37
Well, they can do that because
they are quantum mechanical

272
00:19:37 --> 00:19:40
particles.
We cannot treat those electrons

273
00:19:40 --> 00:19:44
like we treat atoms and
molecules which,

274
00:19:44 --> 00:19:48
for the most part,
behave as classical particles.

275
00:19:48 --> 00:19:51
And, actually,
this is going to be the subject

276
00:19:51 --> 00:19:56
of the first few lectures here,
the need for this new kind of

277
00:19:56 --> 00:20:01
mechanics to explain phenomenon
like this and to explain

278
00:20:01 --> 00:20:04
chemical bonding.
All right.

279
00:20:04 --> 00:20:08
So, these electrons tunnel
right through.

280
00:20:08 --> 00:20:13
What does that mean?
Well, what that means here,

281
00:20:13 --> 00:20:17
for this experiment,
is that if I then take and

282
00:20:17 --> 00:20:22
attach a wire to the tungsten
tip and a wire to the platinum

283
00:20:22 --> 00:20:27
surface and I put an ammeter in
between, I will see a

284
00:20:27 --> 00:20:33
measurement of a current.
There are electrons going from

285
00:20:33 --> 00:20:37
this tungsten tip to this
platinum surface.

286
00:20:37 --> 00:20:41
I measure a current.
Well, that is nice.

287
00:20:41 --> 00:20:45
But now, from that,
how do I get this image of

288
00:20:45 --> 00:20:50
these 28 CO molecules?
Well, what I do is I also have

289
00:20:50 --> 00:20:55
this tungsten tip mounted not
only on a piezoelectric crystal

290
00:20:55 --> 00:21:00
that allows me to go up and
down.

291
00:21:00 --> 00:21:04
But another piezoelectric
crystal, which allows me to move

292
00:21:04 --> 00:21:08
it from side to side with
control on the order of an

293
00:21:08 --> 00:21:11
angstrom.
I can take the tungsten tip and

294
00:21:11 --> 00:21:15
I am going to move it over by a
certain amount.

295
00:21:15 --> 00:21:20
And exactly I am going to know
that certain amount is because I

296
00:21:20 --> 00:21:22
calibrated my piezoelectric
crystal.

297
00:21:22 --> 00:21:26
But now, when I move that
tungsten tip over,

298
00:21:26 --> 00:21:31
what is going to happen to this
current here?

299
00:21:31 --> 00:21:33
Pardon?
It is going to go down.

300
00:21:33 --> 00:21:37
It is going to plummet.
It is going to go to zero.

301
00:21:37 --> 00:21:39
Why?
Because when I move this tip

302
00:21:39 --> 00:21:44
over, I increase the distance
between the end of the tip and

303
00:21:44 --> 00:21:48
the oxygen onto the molecule.
And when I increase the

304
00:21:48 --> 00:21:51
distance, what I do is make this
barrier wider.

305
00:21:51 --> 00:21:56
And the wider the barrier is,
the more difficult it is for

306
00:21:56 --> 00:22:00
those electrons to tunnel
through.

307
00:22:00 --> 00:22:03
And so the current actually
goes down.

308
00:22:03 --> 00:22:08
To compensate for that,
I am now going to take the

309
00:22:08 --> 00:22:14
tungsten tip and move it down by
just enough such that I

310
00:22:14 --> 00:22:19
reestablish the current that I
originally have.

311
00:22:19 --> 00:22:23
And, of course,
I know exactly by how much I

312
00:22:23 --> 00:22:27
moved it down because,
again, I have my tip

313
00:22:27 --> 00:22:32
calibrated.
I've got two points now.

314
00:22:32 --> 00:22:35
I need a third point.
I am going to take that

315
00:22:35 --> 00:22:40
tungsten tip and I am going to
move it over again.

316
00:22:40 --> 00:22:43
Again, the current is going to
go down.

317
00:22:43 --> 00:22:48
But, in order to reestablish
the original current I had,

318
00:22:48 --> 00:22:51
I am going to move this down
further.

319
00:22:51 --> 00:22:56
And, again, I will know how
much I move the tip over and I

320
00:22:56 --> 00:23:02
will know exactly how much I
move the tip down.

321
00:23:02 --> 00:23:05
To get that image,
I am going to provide a color

322
00:23:05 --> 00:23:08
code.
I am going to say that when the

323
00:23:08 --> 00:23:12
tungsten tip is the largest
distance away from the surface,

324
00:23:12 --> 00:23:17
well, then that is going to
show up here on this picture and

325
00:23:17 --> 00:23:20
it is going to show up as a very
light color.

326
00:23:20 --> 00:23:24
That is the highest points.
When the tungsten tip is a

327
00:23:24 --> 00:23:27
little bit closer to the
surface, well,

328
00:23:27 --> 00:23:32
that is going to show up as the
darker colors.

329
00:23:32 --> 00:23:37
And, as it gets lower and lower
and lower, it is going to be

330
00:23:37 --> 00:23:42
deeper and deeper orangey.
Finally, when I am actually

331
00:23:42 --> 00:23:47
tunneling to the platinum
surface, instead of a CO

332
00:23:47 --> 00:23:51
molecule, I am going to make
that a blue color.

333
00:23:51 --> 00:23:56
That is how we get the image of
this molecular man.

334
00:23:56 --> 00:24:02
Now, you do see that these CO
molecules are in the form of a

335
00:24:02 --> 00:24:07
little person.
This does not represent,

336
00:24:07 --> 00:24:12
in the most au courant
language, intelligent design.

337
00:24:12 --> 00:24:17
Rather, this represents the
work of a very patient

338
00:24:17 --> 00:24:21
experimentalist,
i.e., graduate student,

339
00:24:21 --> 00:24:26
who spent 24 hours,
and I know this to be the case,

340
00:24:26 --> 00:24:33
moving these CO molecules into
this particular arrangement.

341
00:24:33 --> 00:24:36
How did he do that?
Well, how he did that is the

342
00:24:36 --> 00:24:38
following.
What he did was took this

343
00:24:38 --> 00:24:43
platinum surface and then opened
up a bottle of CO in the vacuum

344
00:24:43 --> 00:24:46
chamber, or let some CO into the
vacuum chamber,

345
00:24:46 --> 00:24:50
and the molecules just absorbed
anywhere they wanted to,

346
00:24:50 --> 00:24:54
well, sort of anywhere they
wanted to on this platinum

347
00:24:54 --> 00:24:55
surface.
First of all,

348
00:24:55 --> 00:25:00
he had to figure out where the
molecules were.

349
00:25:00 --> 00:25:05
The soccer balls are the CO
molecules and the tungsten tip

350
00:25:05 --> 00:25:10
here is my leg and my foot.
And so the first thing he did

351
00:25:10 --> 00:25:15
was scan the surface in order to
figure out where the CO

352
00:25:15 --> 00:25:20
molecules are and goes,
okay, I know where they are.

353
00:25:20 --> 00:25:26
Then he brought this tip down
right next to one of the CO

354
00:25:26 --> 00:25:30
molecules.
Then he gave that piezoelectric

355
00:25:30 --> 00:25:35
crystal a pulse of voltage which
jerked it and the CO molecule

356
00:25:35 --> 00:25:37
went flying away.
Well, that is nice,

357
00:25:37 --> 00:25:41
but now where is it?
Again, you have got to go scan

358
00:25:41 --> 00:25:44
along the whole surface to find
it.

359
00:25:44 --> 00:25:46
Well, he pushed it over too
far.

360
00:25:46 --> 00:25:50
Now we have got to come over
here, put the tip down,

361
00:25:50 --> 00:25:54
give another voltage pulse
before his tip breaks.

362
00:25:54 --> 00:25:57
Well, you get the idea.
24 hours later,

363
00:25:57 --> 00:26:02
you've got it.
This is the beginnings of

364
00:26:02 --> 00:26:07
nanotechnology.
You can see that it is going to

365
00:26:07 --> 00:26:12
be a long time before
manipulation of individual atoms

366
00:26:12 --> 00:26:16
and molecules like this,
one at a time,

367
00:26:16 --> 00:26:21
before that competes
effectively with synthesis in a

368
00:26:21 --> 00:26:27
beaker where you get the
molecules right where you want

369
00:26:27 --> 00:26:32
them because of chemistry
instead of this mechanical

370
00:26:32 --> 00:26:38
manipulation.
Well, even though 100 years ago

371
00:26:38 --> 00:26:46
these direct observations of
atoms and molecules was not

372
00:26:46 --> 00:26:53
possible, it was by the late
1800s pretty well accepted,

373
00:26:53 --> 00:26:59
or the evidence for the atomic
structure of matter,

374
00:26:59 --> 00:27:05
atoms as the most basic
constituent of matter,

375
00:27:05 --> 00:27:12
that evidence was really pretty
compelling.

376
00:27:12 --> 00:27:14
And, in fact,
by the late 1800s,

377
00:27:14 --> 00:27:20
it was basically believed that
the theoretical structure of the

378
00:27:20 --> 00:27:24
universe was complete.
Nature was understood.

379
00:27:24 --> 00:27:29
There were no big discoveries
to be made yet.

380
00:27:29 --> 00:27:32
And, in fact,
there was some justification

381
00:27:32 --> 00:27:36
for that attitude,
because certainly by the late

382
00:27:36 --> 00:27:40
1800s Newtonian mechanics,
the mechanics that described

383
00:27:40 --> 00:27:45
how bodies all around us moved,
including astronomical bodies,

384
00:27:45 --> 00:27:49
well, that had already been
known for over 200 years.

385
00:27:49 --> 00:27:53
Thermodynamics was formulated
already by that time.

386
00:27:53 --> 00:27:59
Statistical mechanics was also
formulated by that time.

387
00:27:59 --> 00:28:03
Statistical mechanics is a
field that relates the

388
00:28:03 --> 00:28:08
microscopic description of
matter to the macroscopic

389
00:28:08 --> 00:28:12
behavior of matter.
And, very importantly,

390
00:28:12 --> 00:28:17
there were experiments by
Young, Fresnel and Hertz that

391
00:28:17 --> 00:28:23
seemed to put to rest the notion
that light was a particle.

392
00:28:23 --> 00:28:28
Those experiments really
nailed, or seemed to nail the

393
00:28:28 --> 00:28:35
idea of light as a wave,
light has wavelike particles.

394
00:28:35 --> 00:28:40
They verified Maxwell's
equations that unified the

395
00:28:40 --> 00:28:44
fields of optics and
electromagnetism.

396
00:28:44 --> 00:28:50
All of these accomplishments
surely did justify a very proud

397
00:28:50 --> 00:28:56
feeling amongst the scientific
community.

398
00:28:56 --> 00:29:00
And, at that time,
the feeling was that the work

399
00:29:00 --> 00:29:05
that remained was largely to
investigate the next decimal

400
00:29:05 --> 00:29:10
place and that is it.
Well, if you look really

401
00:29:10 --> 00:29:13
carefully, though,
at the evidence,

402
00:29:13 --> 00:29:17
even with all of these
accomplishments,

403
00:29:17 --> 00:29:21
there were beginning to be,
in the late 1800s,

404
00:29:21 --> 00:29:25
some experiments that were
suggesting that,

405
00:29:25 --> 00:29:30
one, maybe the atom was not the
most basic constituent of

406
00:29:30 --> 00:29:35
matter.
That was the first set of

407
00:29:35 --> 00:29:39
measurements that indicated
something was amiss,

408
00:29:39 --> 00:29:44
the fact that the atom wasn't
the most elementary particle.

409
00:29:44 --> 00:29:48
And we are going to look at
these sets of measurements.

410
00:29:48 --> 00:29:53
Second, the other observation
that hinted that this classical

411
00:29:53 --> 00:29:58
thinking as amiss was the
observation of the photoelectric

412
00:29:58 --> 00:30:02
effect.
Because the photoelectric

413
00:30:02 --> 00:30:07
effect, what it did was it
showed that light was behaving

414
00:30:07 --> 00:30:12
like a particle and not a wave,
and that sent a lot of

415
00:30:12 --> 00:30:17
consternation throughout the
scientific community.

416
00:30:17 --> 00:30:21
We are going to look at these
two tracks.

417
00:30:21 --> 00:30:27
And we are going to start by
talking about the fact that the

418
00:30:27 --> 00:30:33
atom is not the most basic
constituent of matter.

419
00:30:33 --> 00:30:38
That at least you can divide
the atom up into an electron and

420
00:30:38 --> 00:30:42
a nucleus.
We are going to start here with

421
00:30:42 --> 00:30:43
this gentleman,
J.J.

422
00:30:43 --> 00:30:46
Thompson.
Remember that name.

423
00:30:46 --> 00:30:50
It is going to come back.
Discovery of the electron.

424
00:30:50 --> 00:30:52
This is 1897.
What J.J.

425
00:30:52 --> 00:30:57
Thompson was interested in
doing was understanding what a

426
00:30:57 --> 00:31:03
discharge was,
or what made up a discharge.

427
00:31:03 --> 00:31:07
For example,
if you have a glass vessel that

428
00:31:07 --> 00:31:12
you evacuate and then you have a
cathode in that glass vessel and

429
00:31:12 --> 00:31:18
you have an anode in that glass
vessel, and you also put some

430
00:31:18 --> 00:31:23
molecular hydrogen in it,
fill it up with molecular

431
00:31:23 --> 00:31:26
hydrogen.
But now what you do is put a

432
00:31:26 --> 00:31:32
negative voltage on the cathode
and a positive voltage on the

433
00:31:32 --> 00:31:36
anode.
And you crank up the potential

434
00:31:36 --> 00:31:39
energy difference,
the voltage difference between

435
00:31:39 --> 00:31:43
the cathode and the anode.
And you keep cranking it up.

436
00:31:43 --> 00:31:47
You have to get really pretty
high, but at some point all of a

437
00:31:47 --> 00:31:51
sudden what happens is that the
gas here begins to blow.

438
00:31:51 --> 00:31:54
And you get the establishment
of this discharge,

439
00:31:54 --> 00:31:56
this plasma.
And J.J.

440
00:31:56 --> 00:32:00
Thompson was just interested in
finding out what was in this

441
00:32:00 --> 00:32:04
plasma.
What he did to investigate it

442
00:32:04 --> 00:32:10
is he punched a hole in this
anode right here and let out a

443
00:32:10 --> 00:32:15
little bit of this plasma.
He let it impinge on a kind of

444
00:32:15 --> 00:32:20
phosphor screen here.
Even though the plasma leaking

445
00:32:20 --> 00:32:26
out was kind of glowing in the
dark, well, it also was glowing

446
00:32:26 --> 00:32:31
when it hit the phosphor screen.
That lit up.

447
00:32:31 --> 00:32:38
But then he took a pair of
parallel metal plates above and

448
00:32:38 --> 00:32:45
below this luminous beam and put
a potential difference on them,

449
00:32:45 --> 00:32:49
some delta V.
And this delta V is just a

450
00:32:49 --> 00:32:57
fraction of what this delta V
is, so it is very small.

451
00:32:57 --> 00:33:01
But what he noticed is that
some of this luminous beam was

452
00:33:01 --> 00:33:06
actually attracted toward that
positively charged plate.

453
00:33:06 --> 00:33:11
And so, if you have got
something that is attracted to

454
00:33:11 --> 00:33:16
this positively charged plate,
what does it mean about this

455
00:33:16 --> 00:33:19
particle?
It is negatively charged.

456
00:33:19 --> 00:33:22
It is just Coulomb's
interaction.

457
00:33:22 --> 00:33:27
And he could measure right here
the amount of deflection from

458
00:33:27 --> 00:33:32
the center line.
I am going to call that amount

459
00:33:32 --> 00:33:38
of deflection delta X sub minus
to indicate that

460
00:33:38 --> 00:33:42
this looks like a negatively
charged particle.

461
00:33:42 --> 00:33:47
Now, Thompson also knew enough
electromagnetism at that time to

462
00:33:47 --> 00:33:53
realize that the amount of that
deflection has to be directly

463
00:33:53 --> 00:33:58
proportional to the charge on
that particle.

464
00:33:58 --> 00:34:01
In other words,
the greater the charge the

465
00:34:01 --> 00:34:06
larger the deflection.
I am going to represent that

466
00:34:06 --> 00:34:11
charge by E sub minus.
He also recognized that the

467
00:34:11 --> 00:34:15
heavier that particle,
the more difficult it is going

468
00:34:15 --> 00:34:21
to be to deflect the particle to
the positively charged plate.

469
00:34:21 --> 00:34:25
That amount of deflection is
going to be inversely

470
00:34:25 --> 00:34:32
proportional to the mass of that
negatively charged particle.

471
00:34:32 --> 00:34:35
But then Thompson did a further
experiment.

472
00:34:35 --> 00:34:40
He increased delta V even more.
And here, I am taking a little

473
00:34:40 --> 00:34:44
liberty with the story.
It is a little bit more

474
00:34:44 --> 00:34:49
complicated, but I am just
trying to get the essence here

475
00:34:49 --> 00:34:52
across.
He cranked this up some more.

476
00:34:52 --> 00:34:56
And then, if you looked really,
really carefully,

477
00:34:56 --> 00:35:01
what happened is he also saw
some of this being deflected

478
00:35:01 --> 00:35:06
toward the negatively charged
plate.

479
00:35:06 --> 00:35:08
Indicating that,
lo and behold,

480
00:35:08 --> 00:35:14
there must also be some
positively charged particles in

481
00:35:14 --> 00:35:18
this luminous beam.
And he called that deflection

482
00:35:18 --> 00:35:25
delta X sub plus.
Again, the amount of deflection

483
00:35:25 --> 00:35:30
for the positively charged
particles has to be proportional

484
00:35:30 --> 00:35:36
to the charge on that positively
charged particle and inversely

485
00:35:36 --> 00:35:44
proportional to the mass of that
positively charged particle.

486
00:35:44 --> 00:35:49
But the other critical
observation that he made was

487
00:35:49 --> 00:35:54
that the amount of deflection
for a given voltage,

488
00:35:54 --> 00:35:59
for that negatively charged
particle was much,

489
00:35:59 --> 00:36:05
much larger that the amount of
deflection for the positively

490
00:36:05 --> 00:36:10
charged particle.
That is the evidence.

491
00:36:10 --> 00:36:15
Now we have to think.
Now we have to make some

492
00:36:15 --> 00:36:19
guesses.
What he guessed is that the

493
00:36:19 --> 00:36:25
positively charged particles
here were H plus.

494
00:36:25 --> 00:36:31
How did he know that?
Well, what he did know is that

495
00:36:31 --> 00:36:35
in this plasma there were some
neutral hydrogen atoms.

496
00:36:35 --> 00:36:39
He knew that.
How he knew that I am going to

497
00:36:39 --> 00:36:43
tell you, or we are going to
talk about in a few days.

498
00:36:43 --> 00:36:48
But he knew that this plasma
takes the H two molecule

499
00:36:48 --> 00:36:52
and tears it apart and makes
hydrogen atoms.

500
00:36:52 --> 00:36:56
And he knew it was neutral.
And so he reasoned that what

501
00:36:56 --> 00:37:01
must be happening is that
something has to be coming off

502
00:37:01 --> 00:37:08
of this hydrogen atom to make it
a positively charged particle.

503
00:37:08 --> 00:37:11
He said, okay,
this is going to be H plus.

504
00:37:11 --> 00:37:14
But then, because this was

505
00:37:14 --> 00:37:18
neutral to begin with,
whatever came off of the

506
00:37:18 --> 00:37:22
hydrogen has to be that
negatively charged particle so

507
00:37:22 --> 00:37:26
that when they come together
they are neutral,

508
00:37:26 --> 00:37:30
because a hydrogen atom is
neutral.

509
00:37:30 --> 00:37:33
And, of course,
ultimately that negatively

510
00:37:33 --> 00:37:37
charged particle was called an
electron.

511
00:37:37 --> 00:37:41
But the key point is that he
said, well, it must be then,

512
00:37:41 --> 00:37:47
when these two particles come
together, you are going to have

513
00:37:47 --> 00:37:51
a neutral particle.
It must be that the absolute

514
00:37:51 --> 00:37:56
magnitude of the charge on that
negatively charged particle,

515
00:37:56 --> 00:38:00
the electron,
that has to equal the magnitude

516
00:38:00 --> 00:38:04
of the charge on that positively
charged particle,

517
00:38:04 --> 00:38:10
this hydrogen plus ion.
That was the conjecture.

518
00:38:10 --> 00:38:17
Well, if that is the case,
if I then take a ratio of delta

519
00:38:17 --> 00:38:24
X minus the delta X plus,
what I am going to get here is

520
00:38:24 --> 00:38:30
just that it will be equal to
the mass of the hydrogen ion

521
00:38:30 --> 00:38:37
divided by the mass of this
negatively charged particle,

522
00:38:37 --> 00:38:42
the electron.
But the observation here is

523
00:38:42 --> 00:38:46
that this deflection,
for the negatively charged

524
00:38:46 --> 00:38:51
particle, is much larger than it
was for the positively charged

525
00:38:51 --> 00:38:53
particle.
That must mean,

526
00:38:53 --> 00:38:58
if this is an equality,
that the mass of this hydrogen

527
00:38:58 --> 00:39:03
ion is much, much greater than
the mass of this negatively

528
00:39:03 --> 00:39:07
charged particle,
the electron.

529
00:39:07 --> 00:39:09
And this is the stunning
result.

530
00:39:09 --> 00:39:13
It is stunning because,
at the time,

531
00:39:13 --> 00:39:18
it was already known that a
hydrogen atom is the least

532
00:39:18 --> 00:39:22
massive atom.
There wasn't evidence for any

533
00:39:22 --> 00:39:27
other atom less massive than a
hydrogen atom.

534
00:39:27 --> 00:39:29
And so here,
in this experiment,

535
00:39:29 --> 00:39:34
what we are finding is that
there is a particle that is less

536
00:39:34 --> 00:39:39
massive than the hydrogen atom.
You can chop the hydrogen atom

537
00:39:39 --> 00:39:42
up.
The atom is not the most basic

538
00:39:42 --> 00:39:46
constituent of matter.
There is some other particle

539
00:39:46 --> 00:39:50
here, which we are going to call
an electron, that is less

540
00:39:50 --> 00:39:55
massive than a hydrogen atom.
That was the first piece of

541
00:39:55 --> 00:39:59
evidence for being able to split
that hydrogen atom,

542
00:39:59 --> 00:40:04
or that atom up into smaller
particles.

543
00:40:04 --> 00:40:08
The first evidence that the
atom was not the most basic

544
00:40:08 --> 00:40:12
constituent of matter.
Now, it took another ten years,

545
00:40:12 --> 00:40:17
the Millikan oil-drop
experiment, for this ratio to

546
00:40:17 --> 00:40:22
actually be measured accurately
and for the mass of the electron

547
00:40:22 --> 00:40:26
to actually be measured
accurately.

548
00:40:26 --> 00:40:30
And what we now know is that it
takes 1,836 masses of the

549
00:40:30 --> 00:40:33
electron to equal the mass of a
hydrogen atom.