The Foundations of Chemistry

In Section 7-4 we showed that Lewis formulas of polyatomic ions can be constructed

in the same way as those of neutral molecules. Once the Lewis formula of an ion is known,

we use the VSEPR and VB theories to deduce its electronic geometry, shape, and

hybridization, just as for neutral molecules.

VALENCE SHELL ELECTRON PAIR REPULSION

(VSEPR) THEORY

The basic ideas of the valence shell electron pair repulsion (VSEPR) theoryare:

Each set of valence shell electrons on a central atom is significant. The sets of valence

shell electrons on the central atomrepel one another. They are arranged about the

central atomso that repulsions among them are as small as possible.

This results in maximum separation of the regions of high electron density about the

central atom.

A central atomis any atom that is bonded to more than one other atom. In some

molecules, more than one central atom may be present. In such cases, we determine the

arrangement around each in turn, to build up a picture of the overall shape of the entire

molecule or ion. We first count the number of regions of high electron densityaround

the central atom,as follows:

1.Each bonded atom is counted as oneregion of high electron density, whether the

bonding is single, double, or triple.

2.Each unshared pair of valence electrons on the central atom is counted as one

region of high electron density.

Consider the following molecules and polyatomic ions as examples.

8-2

Recall that we must take into account

the “extra” electrons on anions and the

“missing” electrons of cations.

8-2 Valence Shell Electron Pair Repulsion (VSEPR) Theory 309

Lewis dot formula:

Formula:

Central atom:

Number of atoms
bonded to central atom:

Number of unshared
pairs on central atom:

Total number of regions
of high electron density
on central atom:

O

O

O S O

2 

OOC

H

H

HHC

H

H

N H

CO 2 NH 3 CH 4 SO 42 

CNC S

2

2

34

4 4 4

4

010 0

The VSEPR theory assumes that

regions of high electron density

(electron pairs) on the central atom

will be as far from one another as

possible.