Timothy Mulligan
CHE 101 (Formerly CHE 111)
July 6, 2004
Molecular Geometry & Hybridization of Atomic Orbitals
So we already discussed chemical bonding in Chapter 9. This be Chapter 10; so we
cannot keep pretending chemical bonding is so simple. The Lewis Model shows us a few
things about bonding. First it tells the type of bond (ionic, covalent, hydrogen). Second
it shows which atoms are bonding with each other in a two dimensional plane.
As you are aware if you have seen Back to the Future, we do not live in a twodimensional world. (Some people might even argue we do not just live in a three
dimensional world. The fourth dimension, of course, is time; this gets into things like
radioactivity and we are not ready for that just yet.) So being that we live in a three
dimensional world, some scientists had to get up on their high horse and point out that
molecules too probably exist in three dimensional planes. What’s more they said that if
people can be so different, but yet still have the same predictable body shapes, then there
should be a way to predict the different body shapes of different ions, atoms, molecules,
etc.
Well maybe it didn’t happen just like that, but you get the point I am trying to
make. Even if you don’t, suffice it to say that there is a way to predict the 3D shape of
molecules. No, it is not locked up in NORAD or some government stiffy’s office—it is
right here for you (in all its glory)!
This glorious method is called the VESPR model. No, this is not the princess
from the movie SpaceBalls. VESPR stands for Valence Shell Electron Repulsion. This
is simply one of those fancy scientific labels. You already know what it means in simpler
terms: the electrons float around the nucleus of an atom; the outer most electrons are
called the valence shell; this shell’s number of atoms mostly determines whether the
entire atom will react, repel, or be neutral to another atom(s).
The VESR model ignores whether atoms are singly, doubly, or triply, bonded.
They seem to all act as single bonds in determining shape. (Well not really but assuming
so is within margin of error.) Also if the atom is not an atom, but is a molecule, then we
can apply the VESPR model to the different resonant structures within it.
So say we have element A (any element really) and element B (any other
element). A is the central atom, B the lateral. B will have a subscript x next to it to tell
us just how many atoms there are of B. For example if there are 2 atoms of B then we
write the model as AB2. If there are 4 atoms of B then we write AB4. And so on and so
forth ad infinitum nausea.
Well what do we do if the central atom is paired or tripleted? Aye, that is a step
or two ahead. You see as far as the VESPR model is concerned there are two types of
molecules in this world: those that have lone central atoms and those that do not.
Remember that!
So like back to the lone central atom (A) with nasty atoms (B2, B3,B4, B5, B6)
trying to get into his pants. They would very much like to just pull the pants (electrons)
right off A and expose his nucleus :-O . But unfortunately they have not heard of game
theory and they all try to get in there and grab a pant leg. You see what really happens is
the electrons repel each other, even though the nucleus of each Bx wants to get her hands
on A’s electrons.
If there are just two B atoms (AB2) then they will realize that the easiest way to
get A’s pants off and avoid the other B is for each of them to go to one side of A and pull
on a different pant leg. This pulls A’s legs down into a karate split “Kee-Yaa.”
A molecule that has had this traumatic experience happen to him is BeCl2 (his
unfortunate new name). So tell us Be my man, how did that feel? “Man it was awful. I
was buzzin’ around minding my own business when these two Chlorides popped out
from behind a dumpster and started pulling my electrons out. Luckily I studied Tang So
Do Moo Duk Kwan Karate. I went into a split, but the Cl just kept holding on—we’ve
been here ever since in a 180 degree angle. Each Cl is directly opposite each other with
me playing monkey in the middle—a linear plane—I mean all they had to do was ask.”
So there you have it. B was attacked by three F atoms (AB3). They jumped on
top of him with lightening speed. He did not know what hit him. Fortunately for him
there were enough players involved that they repelled each other a little. They formed a
trigonal plane with A in the middle. Each F was equidistant from A; this means that if
you connected the dots between each F atom you’d have an equilateral triangle with A in
the middle.
Poor methane. Poor, Poorer C who was minding his own business looking to
form a chain with his own kind, but abundant H ghetto slapped him with another three of
his buddies (that’s four in total or AB4!) Damn, well almost the same thing happened as
when there are three electron rapists—an equilateral triangle almost forms. Except that
last H thug, lets his friends struggle with the legs of C and he gets on top and starts
pummeling C in the face! This turns the whole structure into a three sided pyramid with
A bouncing around a spot relative to the middle of the thing!!
Well, if you thought that was bad enough wait till you hear this story of
degradation. P was hanging out on the stoop in front of the QuickiMart. Along comes 5
Cl (AB5)! Of course I hate to stereotype elements, but as we have seen before Cl is a
troublemaker—I mean “no” means “no” Cl! Cl doesn’t even listen, he and his boys just
grabbing P’s body parts. Turns out they almost form a Tetrahedral like in the last
example. However there is one more Cl here, so this sneaky guy lets the fourth Cl
pummel P in the face, while he sneaks underneath him and goes for the wallet. This is a
three sided pyramid on top and bottom (made by connecting the dots between Cl’s like in
a constellation). Poor P is stuck in the middle. How do you think this makes him feel?
And how does he react?
Oh man! This is the dirtiest thing I think I have ever heard of. Mind you it
doesn’t happen so often, but when it does, oh boy, you are in for a treat . . . AB6.
Innocent
S was disco dancing up a storm until some F atoms pushed their way onto the
floor. They was all like, “What’s up bitch?” and S was all like “Nothin’ sir.” And the F’s
was all like “That’s right bitch, now go make us a sandwhich!” But they weren’t even
hungry. It was just a figure of speech! They jump all over S. What do you think they
form: some sort of pyramidal structure like the trigonal bipyramidal in the last example?
You are close my brotha. ‘Cept this gang is the real deal—look out Bloods and Crypts!
They gots enough juice to form four sided pyramids on top and bottom of S. That is, if
you connected the dots you would see four F’s making a flat square around A and then
there would be one F directly above S and one F directly below. Notice how the shapes
are becoming more symmetrical.
You see my friends. May I call you my friends? The VESPR model is just the
beginning of a wide world of fun and fantastic shapes that can be made with particle
accelerators. If a smartass ever asks you “Hey, do you know what the VESPR model
states?” Just shout back really loud “THE VESPR MODEL BUILDS ON VALENCE
BOND THEORY IN SHOWING THE QUANTUM MECHANICS OF CHEMICAL
BONDING.”
Shouting is always useful in making people think you are smart (except if you are
Howard Dean or homeless). You also one-up the smarty pants by talking about Valence
Bond theory and Quantum Mechanics. Valence Bond theory is simply means that
electrons still orbit their individual atoms in a molecule. If he tells you he already knew
that then you can tell him a thing or two about Molecular Orbital theory which means that
these same electrons also have different levels of orbit around their original atom. Both
of these must be taken into account when determining shape, too.
This gets into Quantum Mechanics which is really just another name for
Molecular Geometry, except it’s more like Atomic Geometry (often inside Molecular
Geometry) ya dig?
Quantum Mechanics is pretty cool. It’s one of the new frontiers of science. A
smart guy could make a career out of denouncing others theories and giving inconclusive
evidence (just make sure your on the right side of the politics involved and welcome to
tenure). Hey, I’ve seen it happen. He could also buy some “frickin’ lasers” and pump
high energy into graphite. This would give him some credence with the grad assistants
perhaps but not the other professors. To do this he might have to write up a bunch of
stuff about new and great uses for such molecules, like nanotubing or a space elevator. If
he was really good he could become some sort of guru, grow his hair long, and start
giving expensive lectures out of a palm spring in the southwest. Hey it’s been known to
happen.
In conclusion, the universe is a big, beautiful, cruel molecule—and we, we my
friends are the atoms, or the protons? or the quarks? or . . . .