Geometry and Polarity of Covalent Molecules

The Lewis structure is not necessarily a good pictorial representation of the three-dimensional
appearance of a molecule. The actual geometric arrangement of the bonds and different atoms is
obtained by using the VSEPR theory described below. The shape of a molecule can affect its polarity.

THE VALENCE SHELL ELECTRON-PAIR REPULSION THEORY

The valence shell electron-pair repulsion (VSEPR) theory uses Lewis structures to predict the
molecular geometry of covalently bonded molecules. It states that the three-dimensional
arrangement of atoms surrounding a central atom is determined by the repulsions between the
bonding and the nonbonding electron pairs in the valence shell of the central atom. These electron
pairs arrange themselves as far apart as possible, thereby minimizing repulsion.

The following steps are used to predict the geometrical structure of a molecule using the VSEPR
theory.

Valence electron arrangements are summarized in the following table:

number  of  valence

electrons

example geometric   arrangement of  electron    pairs   around

the central atom

shape angle between

electron    pairs

2 BeCl 2 linear 180°

3 BH 3 trigonal planar 120°

4 CH 4 tetrahedral 109.5°

Draw    the Lewis   structure   of  the molecule.

Count   the total   number  of  bonding and nonbonding  electron    pairs   in  the valence shell   of  the

central atom.

Arrange the electron    pairs   around  the central atom    so  that    they    are as  far apart   from    each

other   as  possible.   It  is  important   not to  forget  to  take    into    consideration   nonbonding  pairs.