# Do molecules of each person differ

According to VSEPR theory (pronounced "vesper"), molecules have different shapes because their electron pairs orient themselves so as to minimize repulsions. For example, consider the molecule carbon dioxide, which has two oxygen atoms double bonded to a central carbon atom:

O=C=O

The double bonds represent regions of electron density, and the molecule is more stable when high-density electron regions are far apart; therefore, the molecule takes on a linear shape with the C=O bonds 180º from each other.

Now consider the water molecule:

H-O-H

I haven't shown them (because I can't) but the oxygen atom has two lone pairs of electrons as well as the two bonding pairs that form the H-O bonds. These lone pairs prevent the H-O bonds from sitting in a straight line; in fact, they push them into a bent or angular configuration so that the two H-O bonds make an angle of about 105º.

That's what VSEPR theory says about molecules; their shape is the result of electrostatic repulsion between electron pairs.

Valence-bond theory has a different take on the situation: it uses the concept of hybridized orbitals to explain the 3-D shapes of molecules. In VB theory, atomic orbitals (AOs) from different sublevels can be hybridized to form equivalent orbitals for the purposes of bonding.

Consider the carbon atom. In its ground state the carbon atom's 2nd shell electrons look like this:

↑↓...↑..↑

---...---.---.---

2s......2p

Based just on that orbital diagram, you would expect that carbon can only from two covalent bonds (since a carbon atom in its ground state has only two unpaired electrons). Furthermore, you would expect these bonds to be at a 90º angle since we know that the 2p orbitals are oriented that way.

In fact that's not the case. Carbon easily forms four covalent bonds that are oriented at 109.5º to each other. How does it do this? When carbon forms a compound with four other atoms, each of its four valence electrons is moved into an equivalent orbital formed from the previous 2s and 2p orbitals. The orbital diagram looks like this:

↑...↑...↑...↑

---..---..---..---

......2sp³

Because one s-type orbital and 3 p-type orbitals were hybridized to create 4 half-filled equivalent orbitals, this is called sp³ (read: s-p-three) hybridization.

There are other types of orbital hybridization as well. The hybridization of an atom in a molecule can be determined by the number of electron groups around that atom. An electron group consists of a lone pair, a single bond, a double bond, or a triple bond.

2 electron groups / sp hybridization / electron geometry is linear / angle = 180º

3 electron groups / sp² hybridization / electron geometry is trigonal planar / angles = 120º

4 electron groups / sp³ hybridization / electron geometry is tetrahedral / angles = 109.5º

5 electron groups / sp³d hybridization / electron geometry is trigonal bipyramidal / angles = 90º and 120º

6 electron groups / sp³d² hybridization / electron geometry is octahedral / angles = 90º

I hope that helps. Good luck!