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PROFESSOR: OK, a couple of announcements.
Tomorrow will be the Periodic Table test. I provide the
numbers, you provide the letters.
Friday, the contest ends, 5:00 pm.
Get the submissions to me, either over the internet or if
you have something majestic, you have to cart it in with
the riggers.
Get it to my office.
Last day, we had a lesson on the Aufbau principle.
And the Aufbau principle gave us the--
There's still too much talking.
Still too much talking.
I will tolerate zero, zero talking.
When you walk through that door, I'm assuming it's an act
of free will.
And when you walk through that door, we
enter into a contract.
The contract goes something like this: You have certain
expectations of me.
You expect me to come to class prepared.
You expect me to treat you with respect, not to use
vulgar language, not to say things that are insulting,
insensitive.
And I have certain expectations of you.
I expect you to come to class prepared.
I expect that you've done the reading.
And I expect that you're going to sit quietly.
Absolutely quietly.
Because it's my duty to preserve a fertile learning
environment.
If you don't want to learn, I don't care.
But if you impair the ability of your neighbor to learn, I
will take action.
It's very simple.
Very simple.
Where was I?
I think I was talking about the Aufbau principle.
And the Aufbau principle tells us what the filling sequence
of electrons is in a multielectron atom, which
ultimately we rationalized in terms of the four quantum
numbers and the filling in ascending order of energy.
And then we tried to understand what's behind the
Aufbau principle.
And lastly, we took a journey through central Europe in the
1920s and 1930s.
And we met de Broglie, who gave us the
concept of matter waves.
If matter has wave-like properties, the wavelength
would be given by the ratio of the Planck constant to the
Newtonian momentum.
Heisenberg taught us that we can't deal with precision down
to atomic dimensions, and there's a certain degree of
uncertainty or indeterminacy.
And it's given by this relationship here.
And then, lastly, we saw Schroedinger, who wrote the
wave equation saying that if we have wave-like properties
then let's model the atom as a wave, and he gave us this
equation here, which is essentially the equation of
the simple harmonic oscillator.
The simple harmonic oscillator, which allows you
to tell us what's going on with the plucked string.
And it's essentially just this, the double derivative x
double dot plus kx goes to some function.
And the solution of that is a bunch of sines and cosines.
And that's this equation, but dressed up for night-time.
It's a little bit more sophisticated and gives us the
three dimensions, eigenfunctions that
ultimately, we can take and generate the pretty pictures
of the orbitals.
So, today what I want to do is go a little more deeply into
properties.
And if you learn nothing else from me in 3.091, this is what
I want you to retain.
It's that electronic structure dictates properties.
Electronic structure dictates properties.
You know this on your death bed.
It's that simple.
Electronic structure dictates properties.
Then we're going to take 14 weeks and describe electronic
structure, right?
But that's it.
That's it.
So I want to go back to this.
We got into this energy, Aufbau principle because we
saw on the Periodic Table there's some problems here.
If we just go in terms of quantum numbers, we see
there's two elements in n equals 1 shell.
There's 8 elements in n equals 2 shell.
And, if we just kept going, we would expect to find 18
elements in n equals 3 shell, but instead we found only 8.
And that's where the journey began.
Well, there's a way to sort of help remember what the filling
sequence is.
And that rule is encapsulated by the n plus l relationship.
So for equivalent, n plus l values, for equivalent n plus
l values, you fill in ascending n.
Fill in ascending n.
So let's take a look.
So, here's Aufbau.
n plus l rule.
Here's an example where we have 3d.
d means l equals 2.
p means l equals 1. s means l equals 0.
So 5 plus 0 is 5.
4 plus 1 is 5.
3 plus 2 is 5.
So what do I do?
Well, it's saying go in order of ascending n plus l.
And so, you can snake your way through this n plus l, and get
the filling sequence for the whole Periodic Table.
So let's take a look.
We'll start with 1s.
Well, that's easy.
That's hydrogen and helium.
Then we go next to 2s.
There's lithium and beryllium.
And then next goes 2p.
I mean, nobody's going to try to stick 3s ahead of 2p.
That's pretty straightforward.
2p, and that gets us all the way over to neon.
And then we wrap around over here to 3s.
That gets us from sodium to magnesium.
And then we jump to 3p, gets us from
aluminum over to argon.
And then, we don't go to 3d, because 3p is 3 plus 1 is 4.
4s is 4 plus 0 is 4.
So it says we fill in ascending n.
So we go to 4s next.
And that gets us potassium and calcium.
Then comes 3d.
So that gets us scandium over to zinc. Then comes 4p,
gallium to krypton.
5s.
I'm having fun here, I'm going to keep going.
5s gets us over to strontium.
Next comes 4d.
Gets us all the way over to cadmium.
5p gets us to xenon.
Now it's turn 6s cesium, barium.
4f.
We've got to jump down here.
Finally we're going to get to the f shell.
See?
The 4f is in here.
Now, this is an item of convenience.
Strictly speaking, lanthanum is 57, cerium is 58.
So all of these elements belong in here.
But if we were to put this to scale, the Periodic Table
would be way over here.
It wouldn't fit on a page.
So people, over time, have gotten used to just
putting it down here.
But these elements are in the middle here.
So you go 4d, 5p, 6s, then 4f.
Finally we get over to the edge here, to erbium, and then
we jump over here to hafnium, 5d, over to
mercury, 6p, et cetera.
So the n plus l rule gives you the filling sequence in
ascending order.
That's good.
So we've got a nice compact way of grabbing this e
goes to n plus l.
Now let's look at some properties.
We said it's a table of the elements, but
it's a periodic table.
So let's see what this periodicity looks like.
Now, here's a variation of first ionization energy.
So here it is in kilojoules per mole, and here's my pet
unit, the electron volt per atom.
Because here's hydrogen, of 1.6 megajoules per mole, but I
like the 13.6 electron volts because I can remember 13.6.
I can't remember 1312 megajoules per mole,
kilojoules per mole, whatever it is.
13.6 electronic volts.
So here's hydrogen, there's helium.
And then we drop down to lithium, and then we move
across up to neon, and so on.
So you can see 1s, we filled the 1s shell.
Now, we're going to go to shell n equals 2.
Here's 2s.
And then we go to 2p.
And you can even see, look, boron, carbon, oxygen, the
ascending value of the first ionization energy.
And then there's a little jog here at oxygen.
Because here we've got the three unpaired
electrons for nitrogen.
And then there's the oxygen starts to pair, and we
continue to neon, and so on.
3s, 3p, 4s.
So you can see the relationship between that
property, and the place, the element, in
the Periodic Table.
So how do we measure these values?
Well, I thought it might be a good opportunity to look at,
revisit the whole question of gas dynamics, and also
understand the measurement.
Measurement of ionization energies.
And the technique that's used is called photoelectron
spectroscopy.
Photoelectron spectroscopy.
All right.
And it's got a three-letter initialization, PES.
I want to show you a cartoon of how this works.
Actually, I took one -- this is taken from the
text we used to use.
I like the text that we're using now better, but there
are a few things in the other text that were good.
So, we've scanned these few pages and posted them at the
website if you want to go through and read this stuff.
So let's jump over to that.
So here's an example of a terrible drawing.
I look at this and I haven't got the faintest idea how this
thing works.
It's not the artist's fault.
Somebody should have proofread this thing.
What they should be doing is showing something like this.
So, let's see what's going on.
So what we're going to do is, we're going to bombard a
specimen right here.
Which they never show.
I suppose this atom beam is supposed to be colliding with
the photons.
That would be quite an apparatus to
build, let me tell you.
Instead, what you have is, you have the apparatus sitting so
that you've got the material in the center of the chamber.
And you irradiate with photons.
And the photons have very, very high energy, and they
dislodge electrons.
When they dislodge electrons that's the ionization event.
And then what we do is, we measure the velocity of those
ejected electrons.
And if we know the velocity, we know their energy.
We know the energy of the incident energy and by
difference, we calculate the binding energy of the electron
and the atom.
So let's take a look.
I'm going to draw a cartoon here that gives you a sense of
what's going on.
So, here's the n equals 1 shell, or the k shell.
And then I'm going to show, say, a second shell here.
One, two, three.
So this is the l shell, or n equals 2.
And you might say, gee, he's got that wrong.
It's kind of simple, and so on.
It's as complex as it needs to be for the explanation.
Don't let accuracy trump clarity.
The real accurate picture here is too detailed to convey what
I'm trying to convey.
So, this is the specimen, right?
Let's call this the specimen.
And I'm going to irradiate the specimen with some radiation
of high energy.
And so, I generally indicate the photon by a squiggly line
with an arrow.
That's to indicate it's got wave-like properties.
And I'll usually write h nu next to it, to indicate that
it's a photon of energy h nu.
So the photon comes in, and if the energy is high enough, it
will dislodge an electron.
So this electron is dislodged.
It's now ballistic.
It's rendered ballistic.
It's free.
It's ionized.
It's gone.
And what we're going to do is, we're going to take an energy
balance here.
And at some point you may come back and learn
some quantum mechanics.
This photon is annihilated at this collision.
So the photon doesn't act the way an incident electron does
where it loses some of its energy.
All of the energy is lost. So this photon's gone.
All of the energy is given to this electron.
So let's do an energy balance here.
So I can say that the energy of the incident photon, the
energy of the incident photon, is now going to be given to
the electron.
It's now got some kind of kinetic energy.
E kinetic plus the energy it took to pull
it out of the atom.
Plus E, let's call it binding.
E binding
And we know the incident energy of the photon because
we're running the experiment.
So we set the value of the incident energy
of the photon beam.
And then we send this to a detector.
This goes to a detector.
And the detector measures the velocity and ultimately gives
us the energy of the scattered electron.
This is called the dislodged electron, or photoelectron.
It's the electron that was kicked out by the photon.
So it's known as the photoelectron.
Photoelectron.
Pardon me, let's try that again.
Photoelectron energy is measured at the detector.
And then by difference, we get the binding energy.
We get the binding energy.
So this is h nu.
This one here is going to be a 1/2 m b squared, and then by
difference we get the binding energy.
And what kind of photon energies do we need?
We need fairly high energies.
And so typically, we might use wavelengths down around 1
angstrom, which then makes it an X-ray.
And so, over here in Building 13, we have such
instrumentation, and the material scientists
would call this XPS.
X-ray photoelectron spectroscopy.
And if you don't want to blast all of the electrons out of
the specimen and just get the most weakly bound, then you
want a lower incident photon energy, which means a longer
wavelength.
You might come in at around 100 angstroms, which is
ultraviolet.
You know, people, the general public, is so afraid, they
don't know science.
If they hear X-ray they get all panicked.
You know, it's radiation.
Their children are going to die and all
this kind of stuff.
So, what people will do is, they'll say instead of soft
X-ray they'll say hard ultraviolet.
And then the public thinks, oh, as long I wear sunscreen
I'm going to be OK.
So if you use hard ultraviolet, it's called UPS.
That's nothing to do with brown trucks.
It's ultraviolet photoelectron spectroscopy.
And collectively, this is known as PES--
photoelectron spectroscopy.
All right.
So now, let's go back to this schematic.
Now you can see how bad this schematic was that these
photons are coming in as so.
And striking the specimen here, ejecting the
photoelectrons which then are focused and ultimately
measured here at the detector.
So there's the energy balance.
The energy of the incident photon is distributed across
the kinetic energy of the photoelectron plus the binding
energy, and this is how we measure all of these
quantities.
Since you're using very, very high intensity radiation,
there's nothing saying that you're going to
eject only one electron.
You can eject a plurality of electrons.
And the different electrons have
different binding energies.
So you're going to get a set of binding energies, which
means you'll get a set of lines.
You'll get a spectrum of energies.
So this is what the spectrum looks like, and this is, by
the way, just a general way of looking at any spectrum so
that you train you eye to know what to look for.
If I look at any spectrum, I'm going to have a plot of some
kind of intensity, some kind of intensity unit versus some
kind of energy unit, versus some kind of energy unit.
And sometimes, energy increases from left to right.
But sometimes they might put energy increasing from right
to left as you see in these spectrum.
But energy is along here.
And intensity is related to population.
All other things being equal, if I have a specimen that has
two times the concentration of active species, I'm going to
get twice the strength of the line.
So the intensity measures population.
The energy is related to the characteristic binding energy
of a particular electrons, so the energy is related to the
identity of the species.
This is how we can identify certain species.
I've already shown you if you walk in and you see those four
Balmer series lines on a spectrum, you know that's
characteristic of atomic hydrogen.
Nothing else.
So those are energies.
And then, where do we go?
So we have a line here, but we know that real data have a
little bit of a spread to them.
So you don't see discrete lines; you'll see a spread
centered at about the value.
So let's look at these.
These are highly stylized, but here you can see-- here's
boron for example.
So these are the 1s electrons of boron, and they're held at
a binding energy of about 19.3 megajoules per mole.
And then this is 2s, and here's 2p.
Now there's a difference between 2s and 2p, but they're
of the same order.
They're roughly about a megajoule per mole.
There's subtle differences.
And this goes back to Sommerfeld, who said that even
though there are s and p orbitals, there's a circular
orbit and there's an elliptical orbit.
But they're roughly in the same shell and
that's what you see.
But there's a huge difference in going from shell n equals 1
to n equals 2.
This is not to scale.
You see they got a little break there.
And now look, what's the electronic structure of boron?
1s2 2s2 2s1.
There are two 2s electrons, but only one 2p electron.
And this is trying to show that the height here is twice
the height of the 2s peak.
So you see that we've got two electrons here, one electron
here, and it's easiest to pull out the 2p electrons first.
And you go on, there's beryllium and so on.
So we see that we can build data in this fashion here.
We keep going.
Here's carbon, which is 2, 2, 2.
And then oxygen has 2s2--
pardon me.
1s2, 2s2, 2p4.
So this is two electrons, this is four electrons.
Showing that the 2p height is twice the 2s height.
And then finally, neon is 2s2, 2p6.
So this is roughly three times the height of the 2p.
But all of the 2p numbers are roughly of the same order of
magnitude and decidedly smaller than
what's going on at 1s.
And what else do you see?
Well, the carbon has 6 protons, so the inner
electrons of carbon are held not as tightly as the inner
electrons of neon, which has 10 protons.
All of this comes out.
The data are all good.
So now what do we want to talk about?
What's all this about?
I said electronic structure leads to properties.
The first thing we want to talk about is reactivity,
chemical reactivity.
Well it's pretty clear from looking at these XPS data that
those inner shell electrons aren't going anywhere.
They're too tightly held.
So first thing is the only electrons that we can even
think about involving in chemical reactivity must be
the electrons in the outermost shell.
These are called valence electrons.
The outermost shell is called the valence shell.
So let's get that down because that's probably important.
So chemical reactivity determined by only electrons
in outermost shell.
And we're going to term that the valence shell.
And those electrons are called the valence electrons.
And, these are the only ones that we expect to see involved
in chemical reactivity.
And we want to have a measure.
You can see that there are subtle differences between the
energies that hold the valence electrons of carbon as opposed
the valence electrons of neon.
And so what we can do is have a measure of the ability for
valence electrons to react.
That is to say, to participate in chemical activity.
And that measure is called the average
valence electron energy.
And there's no special symbol for that, so we
just call it AVEE.
And it's just the values of the valence
electron energies averaged.
So if I want to take the average valence electron
energy for say, oxygen.
So I can take it right off of there.
I can see I'm going to need two times the ionization
energy of 2s plus four times the ionization energy of 2p.
And those data are up there.
All divided by 6.
2 plus 4, the total number of valence electrons.
And if I go through the math up there I get 1.91 megajoules
per mole, which I prefer to expresses as 19.8 electron
volts per atom.
And when we go through and calculate the values of
average valence electron energies, we find trends.
And here's what the trends look like.
So you have, first of all, following along with the
ionization energies, very similar values.
Pardon me.
So here's the plot of average valence electron energy in bar
heights arranged as the elements are found on the
Periodic Table.
So you have hydrogen here, helium on this scale
would be way up.
And then here's lithium, beryllium, boron.
And so you see a monotonic increase as you move
from left to right.
And you see a monotonic decrease if you stay in the
same column from low mass to high mass.
So what's going on there?
Why do we have that change?
We have similar electronics structures.
We have similar electronics structures in a given column
and lithium is s1, sodium is s1, potassium is
s1, rubidium is s1.
And let's put some values on here.
Let's put some values.
For the majority of elements in the Periodic Table, we get
values of average valence electron energy less than 11
electron volts.
And when we have such values, we have the following
properties: the valence electrons are weakly held.
The valence electrons are weakly held.
How do we know this?
Because the binding energy isn't so strong.
That means the electronic are weakly held.
So we say that the element is a good electron donor.
It's a good electron donor and we term this a metal.
The property that makes an element a metal is that it has
a low value of average valence electron energy and therefore,
it is a good electron donor.
And it turns out about 75% of the Periodic
Table is made of metals.
At the other extreme we have high values of average valence
electron energy.
High values.
So when the average valence electron energy is high, we
have the complementary set of properties.
So that means the valence electrons are tightly held.
Valence electrons tightly held.
Tightly held.
And so the element is a poor electron donor.
But complementary fashion, it is a good electron acceptor
because when it gets near electrons it tends to grab
them and hold on to them.
So this is a good electron acceptor.
And we term such a element a nonmetal.
And so roughly, about 25% of the Periodic Table is
non-metallic.
And I think I've got that shown here.
And you see just a few elements off
to the upper right.
And then in the middle, we've got this--
about a half a dozen elements and they have values of
average valence electron energy intermediate between 11
and 13 electron volts.
And so, they can behave either as electron donors or electron
acceptors depending on who else is in the room.
So if you put an element like silicon with a very strong
metal, silicon will act as an electron acceptor.
If you put silicon in the presence of something that's a
very strong nonmetal, silicon can act as an electron donor.
It has that dual property.
And these elements are called semimetals or metalloids.
And I think I've got some images to portray that.
There's the semimetals.
And if you look carefully at the Periodic Table that you
have, you'll see that there's this red staircase here.
So there's silicon, arsenic, tellurium, antimony and so on
that act as semimetals.
They can behave in both fashions.
All right, so now with this classification scheme, let's
start thinking about chemical reactivity.
And seeing if we can, on the basis of where the elements
are found on the Periodic Table, start
to make some judgments.
Well I know one thing right off the bat that along the
right-hand column, the noble gases are chemically inert.
That much we know.
Noble gases are chemically inert.
I know there's a Nobel Prize out there for getting xenon to
combine with fluorine, and the chemistry textbooks love to
point out these exceptions.
But by and large, the group 8 elements are chemically inert.
So what do we know about their electronic structure?
They all have the same electronic
structure ns 2, np 6.
With the exception of helium.
Helium, because it's 1s 2.
There's no p.
So beside helium, starting with neon, we've got 2s2, 2p6.
Argon is 3s2, 3p6, and so on.
n2 p6, it has 8 p electrons.
It has a full valence shell.
And because the last electrons are p electrons, this is
termed octet stability.
There's something about having a full shell that renders the
system satisfied energetically.
And it doesn't want to react anymore.
So we say octet stability, and if you want to be a wise guy,
parenthetically you whisper helium is due at stability.
But you know, whatever.
All right, so let's make this octet stability a hypothesis
and see how far we can go with it.
So the next one I want to look at is sodium.
Now sodium, it's got the electronic structure 1s 2,
2s2, 2p6, 3s1.
Now, I really don't need to talk about anything up to n
equals 3 because this is not valence electrons.
I just put it up there for completeness, but this is all
we have to worry about.
Now we know that sodium is a metal.
It's a metal.
It's got an average valence electron energy of about 5.2
electron volts, which happens to be the ionization energy.
Because that's trivial; it's only got one electron, so the
average valence electron energy is equal to the
ionization energy.
So it's a metal.
So it's a good electron donor.
But let's think about that.
If we could get rid of that electron in sodium, we could
then turn it into something that has the same electronic
structure as neon.
And why do we want to do that?
Well because there seems to be an energy well there.
If we can make something neon-like, then
it's going to be happy.
So let's do it.
Let's write a reaction sodium to lose an electron and become
sodium plus.
And sodium plus now is this minus 3s1.
So it's isoelectronic with neon, and neon
has the octet stability.
But charge neutrality forbids me to
say, OK, lose an electron.
It won't.
The only way for sodium to lose an electron
is to find an acceptor.
So if you put a good electron donor in the presence of a
good electron acceptor, then the donor can give an electron
to the acceptor and both profit from the transaction.
So, where are we going to find a good electron acceptor?
Well I just told you, the nonmetals are the really good
electron acceptors.
So let's go zooming over to the right-hand edge of that
row and look at chlorine.
We need an electron acceptor.
So choose just for argument's sake, chlorine.
It's got an average valence electron
energy way, way up there.
So let's go.
Chlorine, it has the electronic structure of neon
plus 3s2, 3p5.
So chlorine, if it could acquire an electron, would
then become isoelectronic with argon.
So imagine chlorine plus electron becomes Cl minus and
Cl minus is isoelectronic with argon.
So now I've got two elements.
One that's one electron richer than inert gas structure, and
one that's one electron poorer than inert gas structure.
If I put them together, put them both together in the same
reactor, they will react and electron transfer occurs via
electron transfer.
Chemical reaction occurs.
Chemical reaction occurs through electron transfer.
And what's the purpose of electron transfer?
It's to achieve full valence shell.
Achieve full valence shell occupancy, if you like.
Achieve full valence shell occupancy.
So, now we've got it.
But there's more.
There's more.
What happens after the electron transfer step?
We don't have neutral species anymore.
The sodiums have given up their electrons to become
sodium ions.
And the chlorines have acquired electrons to become
chloride ions.
And these are free.
Let's make this simple.
Let's make it a gas phase reaction.
So sodium vapor and chlorine gas have become sodium ion
vapor and chloride ion vapor.
What do you know happens when you have charges of opposite
value, opposite polarity?
There's a coulombic force of attraction.
So the sodium will attract the chlorine.
So let's put the two together.
So sodium attracts chlorine, but it's not over.
There's more sodiums, there's more chlorines.
So chlorine attracts sodium, which attracts sodium.
Which could attract more chlorines.
Which could attract more sodiums.
And what do you see happening here?
Well we're now starting with a gas phase and we're forming
some giant atomic aggregate.
This is a giant ion aggregate.
What's the state of matter going to be if this thing's
honking big?
It's going to be a solid.
We're going to form a solid starting with those two gases:
sodium vapor and chlorine vapor.
And what about the solid?
What do we see about the atomic arrangement here?
All the sodiums are the same size and all the chlorides are
the same size.
They have the same coulombic forces of attraction in
between them.
Can you see that they're going to form not just a solid, but
they're going to form a solid consisting of atoms in a
regular array?
This is going to be an ordered solid.
And there's a plain Anglo-Saxon word for an
ordered solid, it's called a crystal.
Now look at how far we've come.
Look at how far we've come.
10 minutes ago we hadn't done anything about chemical
reactivity.
We made one observation.
We had the tools of course.
We were ready.
We're ready for discovery.
We had the tools.
We had the average valence electron energy.
We knew that helium, neon, argon, krypton, xenon, radon,
et cetera, all have this inertness.
We postulated that the octet stability was some--
as a sweet spot.
And then we went with it.
And operating with that one postulate, we get to electron
transfer and we're concluding that if I take a really,
really good metal--
see, I can generalize this now.
There's nothing peculiar about sodium that makes it react
this way and no other element will react this way.
I could replace sodium with any other metal.
I could say any metal, any metal will react with any
nonmetal in order to achieve octet stability.
So now you can take big pieces of the Periodic Table and
conclude that entire sets of elements will act in this way
to form ionic compounds, crystals, atoms of regular
periodicity.
That's a nice pun there, right?
We started with the Periodic Table and now we have periodic
spacing of atoms. I think that's beautiful.
Obviously this view is not shared by
the rest of the audience.
This crystal consists of ions.
It's held together by what chemists like
to refer to as bonds.
This is called an ionic bond.
And what you're witnessing here is ironic bonding via
electron transfer.
So we've now categorized the first form of primary bonding.
Sodium chloride, magnesium oxide--
ionic bonding.
And ionic bonding must give us solids at room temperature.
If you've got ionic bonds and you don't have a solid,
clearly you are at elevated temperature where at elevated
temperature, the thermal energy is disruptive enough to
break those bonds.
Those are very, very strong bonds.
And what we're going to do next day is look at the nature
of those bonds.
So I think I'm going to stop the formal
lesson at this point.
And by the way, perhaps I failed to mention on the first
day of the class.
What I'm doing is I'm lecturing for, out of the 50
minutes, about 45 on hard core topics.
And then, about the last 5, 7 minutes, I want to go to
something related to these-- chemistry in the
world around us.
That's why you see this little break.
But it doesn't mean that class is dismissed.
There's a difference between changing topics
and dismissing class.
I don't know how people confuse the two.
But somehow I've inadvertently communicated to you evidently.
So we're not dismissing yet.
We're going to try to apply this knowledge somehow.
So what I wanted to do was to show you what happens with
these ionic solids in commerce.
And in particular, I wanted to talk about metallurgy and the
best form of metallurgy, which is of course,
electrometallurgy.
Which is the kind of research that I'm involved in.
And electrometallurgy is quite pervasive.
The aluminum beverage can is made by electrometallurgy, as
is magnesium.
So I'm going to talk to you a little bit about magnesium.
This is a bar of magnesium.
Obviously, it's magnesium because if it were steel, it
would be so massive that I wouldn't be able to fling it
around like this.
The density of iron is 7.87, the density of magnesium is
about 1.76.
It's less than 2.
It's 2/3 of the density of aluminum.
It's lighter than aluminum.
Now perhaps some of you've been told
that magnesium burns.
Well, let's see what happens.
There's a billet of magnesium and this is
the key made of steel.
We're OK.
It's not going to burn.
That's a myth.
Magnesium in powder form, magnesium in
ribbon form will burn.
The surface to volume ratio is so high that the oxidation
generates a lot of heat.
But here the surface to volume ratio is so
low that it's fine.
In fact, I went to school at the University of Toronto.
And at the time the department head was a man by the name of
Pigeon who invented a process during World War II to make
magnesium cheaply.
And when he came into the classroom to lecture on the
day that he would talk about magnesium, in those days they
had lab benches at the front of the class with sinks and
gas outlets.
And he would put the magnesium billet on a stand and light a
Bunsen burner under it.
And of course, people in the class sort of leaned back, and
he would lecture for 45 five minutes with the Bunsen flame
burning the whole time.
And then towards the end he would extinguish the flame,
let the billet cool.
And then pick it up and go back to his office with it to
make the point.
All right, so how do we make these metals?
Well, what we do is we reverse the electron transfer step.
And we start with the cations and anions, and we force the
electrons back onto the cations anions.
Thereby, creating the neutrals again.
This is called electrolysis.
So cation plus electron gives us neutral.
Anion minus electron gives us neutral.
So in the case of magnesium, we start with magnesium
chloride, which dissolves and forms magnesium cations and
chloride anions.
So, at elevated temperature--
in this case, at about 700 degrees Celsius, the ionic
solid becomes an ionic liquid.
And it's clear, colorless, and has the fluidity of water.
And this is a very simple schematic, so we've made the
cathode of steel negative and on the negative electrode,
magnesium ions are reduced to magnesium, which is a liquid
at this temperature.
And these are little bubbles of magnesium
and they pool here.
And we collect them and syphon them off.
And on the other electrode, we're
making bubbles of chlorine.
And they rise and we collect them.
So we reverse nature.
If you put chlorine in the presence of magnesium it wants
to form magnesium chloride.
Here we do the reverse.
So electrolysis undoes the spontaneous electron transfer
of ion formations.
So I decided I better get something more authoritative
than my little cartoon.
So first I looked on the library website and I found
that there was an article in this volume of Advances in
Molten Salt Chemistry.
And there's this article here, an authoritative article about
chemistry and electrochemistry of magnesium production.
And so I decided to turn to the article and let me read
the first paragraph of this authoritative article.
A cubic kilometer--
what is a cubic kilometer?
It's a cube, 1 kilometer on edge.
A cubic kilometer of sea water contains approximately 1
million tons of magnesium.
1 million tons of magnesium as the salt magnesium chloride
decahydrate dissolved in sea water.
More than has ever been produced in one year by all
the magnesium plants in the world.
The world production of magnesium right now is about
600 million tons.
No, furthermore, sea water contains only 3.7% of the
total magnesium present in the earth's crust. Clearly
magnesium resources are ubiquitous and virtually
inexhaustible.
So when people tell you we've got to recycle because we're
running out of resources, I read this--
I don't know.
You might want to recycle for other reasons, but you can't
make the case it's because of scarcity.
This is magnesium that has been made by this
electrolytic root.
And what's the value of this?
It can substitute for steel in automobiles.
It's density is less than 2.
Steel is about 8.
Factor of 4 you lightweight the vehicle, which means per
unit distance traveled, less fuel consumed.
Fewer emissions.
And less fuel, which means less
dependence on imported petroleum.
So why aren't we doing this?
Why aren't we lightweighting our vehicles with magnesium.
Because the bonds are so strong the energy required to
make this metal is so high that it's very costly.
If I give you a light bulb that costs three times what
the other light bulb does and burns only twice as long, you
won't buy it.
If it costs three times as much and burns 10 times as
long, then you're willing to pay the premium.
So what we do in my research group, among other things, is
study the electrochemical processes by which we make
these metals in order to make the process more efficient.
Thereby reducing the cost, making these lighter materials
more competitive.
Thereby, making the world a better place by electron
transfer in service of humanity.
And I invite you to do the same.
But in order to do so, you have to learn
the lessons of 3091.
Because after all, this is the most important subject you
will take at MIT.
All right, we'll see you on Friday.