Professor Dave again, let’s revisit VSEPR theory.
We’ve learned about valence shell electron-pair repulsion theory, which is a theory that was
developed to explain experimentally observed molecular geometries.
It tells us that the central atom of a molecule can take some number of its atomic orbitals
and hybridize them, making them all of equal energy, or degenerate, and then unpaired electrons
in these hybrid orbitals can form covalent bonds with other atoms given some kind of
orbital overlap.
However, we want to be aware that there are limitations to VSEPR theory.
There are instances in which it does not accurately predict molecular geometry.
Let’s look at some of these instances now.
Take this water molecule.
In the context of VSEPR theory, we would look at the central oxygen atom, with its two bonding
pairs of electrons and two lone pairs of electrons, and with its four electron domains we would
call it sp3 hybridized, exhibiting tetrahedral electron-domain geometry, and bent molecular shape.
Because the lone pairs offer a little more repulsion than bonding pairs, that causes
the H-O-H bond angle to contract a bit from the typical 109.5 degrees we see for tetrahedral
geometry, leaving us at 104.5 degrees.
This is what is observe in nature, so water conforms perfectly to VSEPR theory.
But now, let’s replace oxygen with a larger atom, sulfur.
Being just below oxygen on the periodic table, sulfur also belongs to the group called the
chalcogens, and is thus an analog of oxygen.
It has the same number of valence electrons, and should exhibit similar behavior.
Sulfur will therefore also bond with two hydrogen atoms to form hydrogen sulfide.
VSEPR theory would assess this molecule in precisely the same way as it does water, calling
the sulfur atom sp3 hybridized, and it would predict the same bond angle as it does for water.
But we observe this H-S-H angle to be 92.1 degrees.
Let’s go even further down the group, to tellurium.
This will also behave similarly to oxygen, bonding to two hydrogen atoms to form hydrogen
telluride.
We observe the H-Te-H bond angle in this molecule to be equal to precisely ninety degrees.
So why is this the case?
Well we must understand that in order for VSEPR theory to apply, the EPR part must be relevant.
That is the electron-pair repulsion.
This theory works well when the central atom is on the smaller side, like when its valence
electrons are in the n = 2 shell, such as carbon, nitrogen, and oxygen.
Due to the smaller size of these atoms, the valence electrons are close together and will
generate sufficient repulsion such that hybridization of orbitals is necessary to maximize the distance
between groups.
But the farther out we go, or the greater the atomic radius of the central atom, the
farther away the valence electrons will be from one another, and the less pronounced
the repulsion will be.
If we look at hydrogen telluride, its valence electrons are in the n = 5 shell.
That’s quite far away from the nucleus.
Because of this, its 5p orbitals with unpaired electrons will simply participate in covalent
bonds as they are, without hybridizing.
As we can see from the orbital diagram, one 5p orbital is already filled up, so this half-filled
one will make a sigma bond by overlapping with hydrogen’s 1s orbital, and this other
half-filled one will do the same thing with another hydrogen atom.
Because p orbitals sit precisely on the three axes, they necessarily make ninety degree
angles, and this explains the geometry of hydrogen telluride.
Tellurium exhibits no hybridization whatsoever in forming this molecule.
Looking again at hydrogen sulfide, its valence electrons are in the n = 3 shell.
So the electron pairs are far away enough that the repulsion is minimal, but close enough
that it is not negligible, so the sulfur atom will invoke a slight amount of hybridization.
That’s why the bond angle is close to ninety, but not quite equal to ninety.
So we can see that for molecules with large central atoms, VSEPR theory quickly goes out
the window.
But since organic molecules are filled with carbon, nitrogen, and oxygen, which are all
in the second period, this theory still has abundant application in chemistry.