Chemistry, Third edition

SHAPES OF MOLECULES

Remember that in all the shapes, the electron pairs are as far apart from each other

as possible.

Molecules with lone pairs

In all the examples in Table 5.1, the electron pairs are shared between atoms – they

arebonding pairs. Some molecules have pairs of electrons that are not shared by two

atoms. These electrons are called lone pairs.

The order of repulsion between electron pairs is:

lone pairs repel

more than

lone pairs repel

more than

bonding pairs repel

lone pairs bonding pairs bonding pairs

67

Shapes of

molecules

Using the Lewis structure

for the following molecules

or polyatomic ions,

consider the number of

bonding pairs around the

central atom, then

determine the shapes of

the molecules:

(i) BCl 3

(ii) SiF 4

(iii)CCl 2 F 2 (C is the central

atom)

(iv)[BF 4 ]

(v) [PF 6 ].

Table 5.1Shapes of covalent molecules Exercise 5D

Number of Shape of molecule Bond Examples
electron pairs with respect to the angles
in outer shell electron pairs
of central atom

2 linear 180° BeCl 2

3 trigonal planar 120° BF 3

4 tetrahedral 109° 28 CH 4

5 trigonal bipyramidal 120° and PF 5
90°

6 octahedral 90° SF 6

Example 5.2

What is the shape of the ammonia molecule?

Answer

The Lewis structure of the ammonia molecule is