VSEPR: Valence Shell Electron Pair Repulsion

  The molecular structure of molecules may be predicted with surprising accuracy using VSEPR concepts. The chief tenet of the theory is that electron groups (either lone pairs or bonding pairs of electrons) will try to maximize the distance between themselves and minimize repulsions between electron pairs. Doing so leads to the formation of 5 primary electron group geometries which the molecular structures of the molecules are based on.

Steps for predicting the molecular geometry about a particular atom. (to go directly to the models/examples, click here)

(1) Determine the best Lewis Structure

(2) Identify the number of electron groups present around a particular atom

1. Each lone pair of electrons counts as one electron group
2. Each bonded atom counts as one electron group (regardless if bonds are single or multiple)

 

(3) Establish the "Electron Group Geometry" based on the number of electron groups

This geometry is the the lowest energy arrangement of the electron groups that minimizes electron pair repulsions

# Electron Groups	Electron Group Geometry	Drawing	Ideal Angles
2	linear	"linear"	180°
3	trigonal planar	"trigonal planar"	120°
4	tetrahedral	"tetrahedral"	109.5°
5	trigonal bipyramidal	"trigonal bipyramidal"	90°, 120°
6	octahedral	"octahedral"	90°

(4) Place lone pair electrons in positions that minimize the # of 90° lone pair-lone pair interactions

For trigonal bipyramidal geometry, the lone pairs always occupy the equatorial positions

(5) Place atoms (bonding pairs of electrons) in remaining positions

(6) Assign the Molecular geometry (shape of the molecule) by describing the location of the atoms.

(7) Note any distortions from the ideal bond angles.

Lone pairs require more room and will force atoms (bonding pairs) closer together than expected based upon the electron group geometry

For examples of each molecular geometry and how it is formed,
click on the appropriate # of electron group

Number of Electron Groups	Electron Group Geometry	Number of Lone Pairs	Molecular Geometry	Ideal Bond Angles	Hybridi zation	Example
2	linear	0	"linear"	180°	sp	HCN
Number of Electron Groups	Electron Group Geometry	Number of Lone Pairs	Molecular Geometry	Ideal Bond Angles	Hybridi zation	Example
3	trigonal planar	0	"trigonal planar"	120°	sp2	BF3
3	trigonal planar	1	"bent"	<120°	sp2	SO2
Number of Electron Groups	Electron Group Geometry	Number of Lone Pairs	Molecular Geometry	Ideal Bond Angles	Hybridi zation	Example
4	tetrahedral	0	"tetrahedral"	109.5°	sp3	CH4
4	tetrahedral	1	"pyramidal"	< 109.5°	sp3	NH3
4	tetrahedral	2	"bent"	< 109.5°	sp3	H2O
Number of Electron Groups	Electron Group Geometry	Number of Lone Pairs	Molecular Geometry	Ideal Bond Angles	Hybridi zation	Example
5	trigonal bipyramidal	0	"trigonal bipyramidal"	90°, 120°	sp3d	PF5
5	trigonal bipyramidal	1	"see-saw"	< 90°, <120°	sp3d	SF4
5	trigonal bipyramidal	2	"T-shaped"	< 90°	sp3d	ClF3
5	trigonal bipyramidal	3	"linear"	180°	sp3d	XeF2
Number of Electron Groups	Electron Group Geometry	Number of Lone Pairs	Molecular Geometry	Ideal Bond Angles	Hybridi zation	Example
6	octahedral	0	"octahedral"	90°	sp3d2	SF6
6	octahedral	1	"square pyramidal"	< 90°	sp3d2	BrF5
6	octahedral	2	"square planar"	90°	sp3d2	XeF4