Chemistry, Third edition

SHAPES OF MOLECULES WITH MULTIPLE BONDS 69

Shapes of molecules with multiple bonds

How do we work out the shape of a molecule when it contains double, or even triple

bonds? We can still use VSEPR theory, but a double or triple bond is treated as

though it were a single bond, that is, one bonding pair of electrons. Carbon dioxide

has the structural formula:

O=C=O

Although each double bond contains two shared pair of electrons, they are held

together in the same place that a bonding pair of electrons would be held in a single

bond. Therefore, if each double bond is treated as a single bond, the carbon atom has

the equivalent of two bonding pairs of electrons around it and the shape of the

molecule is linear.

5.3

Shapes of molecules with lone pairs

By first drawing Lewis structures of the following species, then considering the number and

nature of the electron pairs around the central atom, predict the shapes of the following

molecules:

(i) H 2 S (iv)OF 2

(ii)PH 3 (v) SCl 2.

(iii)IF 5

Exercise 5E

Example 5.4

What is the shape of ethyne (common name acetylene)?

Answer

Here the triple bond and single bond on either side of the carbon atoms are

regarded as single bonds in order to work out the overall shape of the molecule.

The molecule is linear.

Example 5.5

What is the shape of sulfur dioxide, SO 2?

Answer

For the purposes of working out the shape, the central sulfur atom acts as if it

has three electron pairs around it, one of which is a lone pair. The molecule is