In Valence Shell Electron Pair Repulsion (VSEPR) theory, pairs of electrons that surround the central atom of a molecule or ion are arranged as far apart as possible to minimize electron-electron repulsion. This simple idea can be used to predict the shapes of molecules by following a simple procedure

· Decide which is the central atom in a molecule.

• In cases of ambiguity, pick the least electronegative atom as this atom will be better able to share its electrons with the other atoms in the molecule.

· Count up the valence (outer shell) electrons on the central atom.

· Count up the electrons used by the outer atoms to make bonds with the central atom.

· The sum of (2) + (3) divided by two gives the Valence Shell Electron Pair (VSEP) count.

· The predicted geometry of the molecule is based on the number of VSEP.

VSEPR	Shape		Example	Hybridization
2	Linear		Carbon dioxide	sp hybridization
3	Trigonal Planar		Formaldehyde	sp^2hybridization
4	Trigonal Pyramidal		Ammonia	sp^3hybridization
5	Bent		Nitrate ion	sp^2 hybrization
6	Tetrahedral		Methane	sp^3hybridization
7	Trigonal Bipyramidal		Phosphorus pentachloride	sp^3d^1 hybridization
8	SeeSaw		Sulfur tetrafluroide	sp^3d^1hybridization
9	Octahedral		Sulfur hexaflouride	sp^3d^2 hybridization

For a comparison of these shapes click on the following link.

The following chart summarizes the shapes and hybridization of certain compounds

Molecule	# of electron pairs	Shapes with, and without non-bonding e pair	Hybridization of central atom
BeH2	2	linear, linear	sp
BF3	3	trigonal planar, trigonal planar	sp^2
CH4	4	tetrahedral, tetrahedral	sp^3
NH3	4	tetrahedral, trigonal pyramidal	sp^3
H2S	4	tetrahedral, bent	sp^3
PF5	5	trigonal bipyramidal, trigonal bipyramidal	dsp^3
BrF3	5	trigonal bipyramidal, T-shaped	dsp^3
TeCl4	5	trigonal bipyramidal, Seesaw	dsp^3
SF6	6	octahedral, octehedral	d^2sp^3
XeF4	6	octahedral, square planar	d^2sp^3
XeF2	5	trigonal bipyramidal, linear	dsp^3

Click on the link below to test your knowledge of molecule shapes and hybridization

Click on the link below for a paper and pencil quiz on the V.S.E.P.R