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ROFESSOR: And so, another way to interpret how well these electrons
like to be about an atom is to define something called the average valence
electron density-- er, energy.
I'll get to the density later. Average valance electron [energy], or AVEE.
AVEE is just like it says. It says, let's look at all the valence electrons.
So for carbon, that would be the 2s electrons and the 2p electrons.
Count them up where, in this case, it's a is the number of s electrons, b
is the number of p electrons, and these are the ionization energies from
those orbitals, the s and p. So for carbon--
actually, I think I did one here for nitrogen. This is an example.
Let's do nitrogen. So nitrogen, again, is 1s 2, 2s 2, 2p 3.
The valence electrons are just these, and you can look at that chart, and
you'll see that the AVEE is found-- well, I got two s electrons.
The ionization energy from the nitrogen, 2s-- nitrogen is 2.45 megajoules per mole.
I got three 2p electrons, and I have to go back to the
chart and look at the-- here you go, 2p is 1.4.
Is that right? Yes.
1.40. It's really hard to read from down here.
Probably hard to read them out there, too. And then I've got to total them up.
Well, I've got five electrons. I'm going to average.
It's the average energy. And you get 1.82 megajoules per mole.
Or if you did that in eV, it's about 18.9. I got to do a better job with my eights, I
know. And my nines.
18.9 eV's. And this is interesting, because if you do
this now averaged out overall, that's a couple periods of the periodic table.
You can see that the average valence electron energy increases across each
period of the table. A few little jogs, but it's generally smooth
across there. And so it fits this picture that what you're
trying to do is fill this subshell, or fill this shell of electrons.
That's where the octet rule comes from. And you've got this chart.
This is that chart in your text. It's just got the ionization energies.
Of course, on the periodic table that you use for your quizzes, it's got the
same information except it's an eV. Oh, this one, kilojoules per mole.
And you can see across here, generally speaking these guys have the highest
ionization energies, and these guys have the lowest ionization energy.
It increases as you move across the periodic table.
Very important to understand jogs in them, but if you stand back and look
at it, the things on the right are harder to get electrons from than
things on the left. Now, there's another property that we're going
to talk about, and this is called electron affinity, which is similar
but not exactly the same. So, we're going to add electrons without changing
Z. STUDENT: Professor?
What do we use the average valence electron energy for?
What does it tell us? PROFESSOR: Yeah.
It's generally saying how well do these electrons-- how well are they held in an average way around
a particular element. STUDENT: So it's just used for comparative
purposes? PROFESSOR: Just used for comparative purposes.
The absolute value-- basically, if you multiply this by the number
of electrons, it's the total energy required to rip off the valence electrons,
is basically what it is, multiplied by the number of valence electrons.
STUDENT: What if the valance electrons are in the s and p orbital?
PROFESSOR: Like a d orbit? STUDENT: Yeah.
PROFESSOR: Same thing. So what you would do is you'd modify this
to put c times the ionization energy from the d orbital.
Good question. It's just all the valence electrons, if they
include d orbitals, just average out all this energy.
Now, you have to know the energies of each of these orbitals, so that's why
you have to do something like the photoelectron spectroscopy.
You have to be given that, or do the experiment, or something like that to
calculate it.