Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

The basis of the VSEPR theory is the fact that pairs of valence electrons in bonded atoms repel one another.  This repulsion pushes electron pairs as far away from each as possible and causes terminal atoms to assume certain orientations about the central atom to which they are bonded.  The resulting arrangement of the atoms in a molecule gives the molecule its distinct geometric shape.

Electron-Group Geometries

The name of this theory implies repulsion between electron pairs, but it is more general than that.  Instead of talking about electron pairs we should talk about electron groups or electron density regions.  An electron group is any collection of valence electrons localized in a region around a central atom that exerts repulsions on other groups of valence electrons. An electron group could be a single pair of electrons either bonded or nonbonded, two pairs of electrons in a double bond or three pairs of electrons in a triple bond.

The most common numbers of electron groups distributed around a central atom are 2, 3, 4, 5, and 6.  The orientations assumed by these electron groups, called the electron-group geometry, are

• 2 electron groups:    linear
• 3 electron groups:    trigonal planar
• 4 electron groups:    tetrahedral
• 5 electron groups:    trigonal bipyramidal
• 6 electron groups:    octahedral

If all of the electron groups above are bonding ones then the geometry of the molecule will be the same as the electron group geometry.  But it is important to keep in mind that the geometry of a molecule is determined by the location of atoms not location of electron groups, although they are sometimes the same. 

VSEPR Notation

In VSEPR notation the molecule is represented by a formula using the letter A for the central atom, X for terminal atoms and E for lone pairs of electrons. The table below uses this notation and shows the geometry of various combinations.  

VSEPR Table

No. of Electron Groups	Electron Pair Geometry (Bond Angle)	No. of lone Pairs	VSEPR Notation	Molecular Geometry	Example
					Formula	Image
2	linear (180o)	0	AX2	linear	BeH2
3	trigonal planar (120o)	0	AX3	trigonal planar	BF3
		1	AX2E	bent	SO2
4	tetrahedral (109.5o)	0	AX4	tetrahedral	CH4
		1	AX3E	trigonal pyramidal	NH3
		2	AX2E2	bent	H2O
5	trigonal bipyramidal (90o, 120o)	0	AX5	trigonal bipyramidal	PCl5
		1	AX4E	see-saw	SF4
		2	AX3E2	T-shaped	BrF3
		3	AX2E2	linear	XeF2
6	octahedral (90o)	0	AX6	octahedral	SF6
		1	AX5E	square pyramidal	BrF5
		2	AX4E2	square planar	XeF4