Chemistry, Third edition

68 5 · MORE ABOUT BONDING

Example 5.2 (continued)

Because lone pairs are not shared between two positive nuclei, they are closer to

the central nucleus than bonding pairs and repel nearby electron pairs more than

bonding pairs. In the ammonia example, there are four electron pairs around the

central nitrogen atom but the angle between the N–H bonds is not 109°28but

107°. This lower angle between the N–H bonds is a result of the lone pair

pushing the bonding pairs closer together; the lone pair repels the bonding pairs

more than they repel each other. The shape of the ammonia molecule with

respect to the electron pairsis a distorted tetrahedron. The structure is shown in

Fig. 5.1.

Comment

The ammonia molecule is also described as trigonal pyramidalwith respect to its

N–H bonds because it looks like a three-cornered pyramid.

Fig. 5.1The structure of the ammonia molecule.

Example 5.3

What is the shape of the water molecule?

Answer

The Lewis structure for water is

There are four electron pairs around the central oxygen atom: two bonding pairs

and two lone pairs. Again, the shape of the water molecule with respect to the

electron pairs is distorted tetrahedral, but the repulsive effect of two lone pairs is

to push the O–H bonds together so that the bond angle between them is 105°.

The structure is shown in Fig. 5.2.

 Comment

The water molecule is described as ‘bent’ or ‘V-shaped’with respect to its O–H bonds.

Fig. 5.2The structure of the water molecule.