We've talked about one theory before, we've talked about VSEPR theory.
Remember VSEPR means: valence shell electron pair repulsion and this has to do with molecular
geometry.
When we draw a compound, the lone pair electrons want more space for themselves, so they push
all the other bonds away from them.
When we're doing electronic geometry and molecular geometry and drawing Lewis dot structure,
we're following VSEPR theory.
Now we're going to talk about the second type of theory, atomic theory or molecular orbital
theory.
We're going to say in the molecular orbital theory electrons are seen as being delocalized,
or spread out over molecule instead of concentrated in a covalent bond.
Let's say we have a compound, hydrogen connected to Cl.
Their forming a covalent bond with each other and realize that when it comes to a covalent
bond, we have to see it as this, we have two forces basically fighting against each other.
We're going to have a bonding orbital is the region of high electron density between nuclei
where a bond forms.
The bond that we're forming is forming because we have bonding orbitals connecting together.
We're going to say opposed to that, fighting against the bonding orbital, is the anti-bonding
orbital.
This is the region of zero electron density between the nuclei where a bond cannot form.
What this means is that every time we're forming a covalent bond between nonmetals, there are
forces that want to keep the bond together and at the same time there are forces that
exist that want nothing but to destroy that bond.
These two bonding forces are basically fighting against one another.
If bonding forces are greater, the bond forms.
If anti-bonding forces are greater, the bond doesn't form.
If they're equal the bond doesn't form.
We want the bonding forces to always be greater so that a bond can be made.
How do we determine who's greater?
The way we determine is by using these and these are our molecular orbital diagrams.
I know it sucks, it looks crazy, but you have to memorize them.
It's similar to the electron configuration that we learned earlier, except now we're
doing molecular orbital electron configurations.
But if you remember how to do electron configuration, this is very similar to it.
We're going to say the MO on the left side has atomic orbitals and molecular orbitals.
The part that we're really concerned with are the molecular orbitals right here.
These atomic orbitals on the left on the right to it, those come from the electron configurations
which we use earlier.
But it's the central parts of each of these MO diagrams that we're concerned with.
Here's the thing, we're going to say the one on the left deals with hydrogen, H2, to N2
on the periodic table.
So hydrogen all the way to nitrogen, deals with the one on the left.
The one on the right deals with O2, F2 and Ne2.
Basically, for these molecular orbital diagrams, we're dealing with diatomic molecules.
Remember, we know that there is only certain types of elements that can be diatomic molecules.
Have No Fear Of Ice Cold Beer.
That's the sentence we learned to remember the diatomic molecules.
Here's the thing, when we were doing MO theory we're going to treat all these elements whether
they're diatomic or not, because neon normally isn't diatomic, but for the molecular orbital
diagram, we treat it as though it's diatomic.
This is the way it's going to work.
The way we look at this is, technically we start out with s1s because electron configuration
begins with one s, but here this molecular orbital diagram shows us 2s at the beginning.
This s here means sigma, so sigma 2s.
Here we have an s with a star, so this is sigma star 2s.
When we see a star, that star signifies we have anti-bonding, so a star means anti-bonding.
Here the 2s, molecular orbital is a bonding orbital and the sigma 2s molecular orbital
is anti-bonding orbital.
We then go up to p, 2p.
This p means pi.
We have a pi 2p molecular bonding orbital, then we have sigma 2p bonding orbital, then
we have a pi star, which means anti-bonding, orbitals and a sigma star 2p anti-bonding
orbital.
We're going to do just like we do with electron configuration to solve this.