So we're still trying to figure out the-- really what
the crux of optical behavior is.
And at this point, we know enough.
We've asked enough questions to kind of lead
us to believe that this model we've
used a little bit-- and this just helped us out--
where the atom, as a sphere, is insufficient.
So it was useful for some mechanical analogies,
for some other purposes.
Now, even if you extend that out to a string of spheres,
as you do for a polymer, I often just
draw it as a squiggly line or something.
It still didn't really explain interaction with light.
Or in fact, really, with any electromagnetic radiation.
So we need to get inside.
We need to crack this thing open to get inside the sphere.
All right.
So let's do that.
And I think that we can proceed fairly quickly
assuming a certain level of background knowledge.
So inside the atom, we've got a nucleus.
And the nucleus contains protons, which are positively
charged, and neutrons, which are-- correction, which
are net neutral to charge.
As the name implies.
Right?
Neutral charge, but this is the most of the mass of the atom.
Essentially, all of it.
I mean, 1,000 times or so lighter
than that is the electron.
So most of the mass of the atom comes from the nucleus.
And then orbiting around the nucleus are electrons,
and the electrons have a negative charge.
And in fact, that's the negative.
That charge that I'm referring to here-- this positive charge,
negative charge-- these are the same quantity,
but opposite signs.
And those are equal to 1.602 times 10
to the minus 19 coulombs.
That's the fundamental charge.
The fundamental charge.
And the nucleus-- particularly, in fact,
the number of protons-- is what determines the element.
It's like the social insurance number for the element.
So sometimes, we refer to the "proton number"
or the "atomic number."
OK.
I actually kind of like the name "proton number."
Depending on where you are exactly,
"atomic number" is probably a little bit more commonly used,
but they're generally understood by I think most people to be
referring to the same thing.
And we use the letter zed-- uppercase letters zed
to denote the atomic number.
So that determines the element.
For example, carbon has an atomic number of 6.
And that's carbon.
You can also have a differing number of neutrons,
and that can create-- what that does
is that creates what we call "isotopes."
Differing number of neutrons.
So they'll have a different mass,
but they've got the same number of protons.
Same proton number, but they have a different mass.
And so a different number of neutrons
creates different isotopes.
So an example of that would be Carbon-13.
OK.
Well, it's carbon.
So it's got to have 6 protons.
Otherwise, it's not carbon.
But if it's at a mass of 13 with these mass units-- in fact,
sometimes, they're called "atomic mass units."
And it's going to have 7 neutrons.
And the first look at the atom that we're going to do,
which is somewhat useful, has a central nucleus--
a central nucleus with electrons in specific shells.
Oh my goodness.
Let me try that one more time.
Specific shell.
I'm trying to get these things to be kind of concentric.
These shells surrounding the nucleus,
and you could have an electron in one of those shells.
Electron in a shell.
OK.
And the model that we're looking at now is-- really,
it's just for-- technically, it's
just hydrogen. This is called the "Bohr model."
I'm going to go, and I'm going to make this.
OK.
The Bohr model after Niels Bohr.
And it has the central nucleus with electrons orbiting
in shells, and one of the features of this model
is that the distance of the electron from the nucleus
is limited to specific values.
So n equals 1, n equals 2.
Look at that.
I erased a space, and then I realized
I didn't make it big enough to fit my writing in.
N equals 3.
So these are these different shells.
Different distances, but they are defined.
This is an important feature.
Defined distance from the nucleus.
Distance from nucleus.
OK.
And so this model is-- I mean, it's not entirely correct.
There's limitations to it.
There's problems with it.
But again, it's a model, and so we'll
see it's actually useful for certain discussions.
In fact, one thing that's actually
quite use-- it's quite useful for discussing
is-- what if you could cause an electron
to move from this shell here closest to the nucleus
to a shell at a further distance?
Or in fact, you could raise the question
of-- well, what if I wanted to move here halfway between n
equals 2 and n equals 3?
Well, the Bohr model tells us, because these
are defined distances, that you can't do that.
You can't move part way.
And in fact, it tells us that you
have to put some energy in to the system
to cause an electron to be excited.
So that would be so-called "excited electron."
And that was energy in.
How could you put energy in?
Well, you could put energy in in the form
of electromagnetic radiation.
E equals hc over lambda.
So you could put some radiation in, excite an electron.
And the other thing that you could
do is-- if you had an atom where you had excited an electron--
and what I'm going to do is I'm going
to leave a little kind of hole there to show-- actually,
I'm going to take that away.
It might be confusing.
So here's the electron, and this is the excited.
It's having so much fun.
It's excited.
Sorry.
The excited electron's been increased in energy level.
It's in this temporary high energy at n equals 2,
and it's only temporary.
It's going to want to drop down to achieve a lower energy.
And so what happens is it drops back down,
and it's only this specific value.
Right?
We said that these defined distances--
what that tells us is that-- another way of saying that
is that the energy is quantized.
Quantized.
"Quantized" means that it can only take on specific values.
Letter grades A, B, C, D are quantized whereas percentage
is more continuous.
So if this excited electron drops back down,
it gives off energy.
OK.
And because it's quantized, it's a little packet of energy,
if you will.
It's a packet.
It's like a specific quantity of energy.
We call that a "photon."
So the Bohr model is useful for discussing
certain things, particularly transitions in energy levels,
to either give off if an electron drops down
to lower energy or, if energy goes in, to excite an electron.
The Bohr model is useful for describing those,
but it also has its limitations, which we'll get to shortly.