VSEPR

The term VSEPR is actually an acronym for __v__alence __s__hell __e__lectron __p__air __r__epulsion.

The concept is simple: electron pairs (as either bonds or lone pair electrons) take up space about a central atom. They are going to position themselves as far from other electron pairs as possible (hence repulsion).

Imagine inflating balloons, and then tying them together by their ends. As you tie more balloons together, their arrangements change to make room.



This concept is a really good model for what happens around atoms in a molecule.

The following chart summarizes the 3D geometries of atoms (X's) or lone pairs (E's) around a central atom (A's).



Here are some animated examples, from Chemmybear.com.

**VSEPR Model for Water**

Water's Lewis diagram is:



To use VSEPR theory, focus on one central atom - in this case, oxygen - at a time. The oxygen atom has two sets of lone pairs, and two bonds, for a total of four electron pairs. Its designation is therefore AX 4.

There are two geometries to determine - an electron pair geometry and a molecular geometry.

Because water is AX 4, its electron pair geometry is "tetrahedral".
 * The electron pair geometry takes both bonds and lone pairs into consideration.
 * The molecular geometry then looks at the shape the atoms in the molecule will take based on the electron pair geometry.

But water has two lone pairs (E's in the chart above), so its molecular geometry is "angular".

Here is the 3D model of water, based on its VSEPR model:

VSEPR models can be used to predict bond angles and orbital hybridizations.


 * = General Structure: || # of Lone Pairs: || Electron Pair Geometry || Molecular Geometry || Bond Angle || Hybrid-ization ||
 * = AX2 || 0 || linear || linear || 180º || sp ||
 * = AX3 || 0 || triangular planar || triangular planar || 120º || sp2 ||
 * = AX2E1 || 1 ||  || angular ||   ||   ||
 * = AX4 || 0 || tetrahedral || tetrahedral || 109.5º || sp3 ||
 * = AX3E1 || 1 ||  || triangular pyramidal ||   ||   ||
 * = AX2E2 || 2 ||  || angular ||   ||   ||
 * = AX5 || 0 || triangular bipyramidal || triangular bipyramidal || 120º, 90º || dsp3 ||
 * = AX4E1 || 1 ||  || seesaw ||   ||   ||
 * = AX3E2 || 2 ||  || T-shaped ||   ||   ||
 * = AX2E3 || 3 ||  || linear ||   ||   ||
 * = AX6 || 0 || octahedral || octahedral || 90º || d2sp3 ||
 * = AX5E1 || 1 ||  || square pyramidal ||   ||   ||
 * = AX4E2 || 2 ||  || square planar ||   ||   ||

Water, having an AX2E2 configuration, will have bond angles of about 109.5º and an sp3 hybridization for oxygen.

Try these problems: