Chemistry, Third edition

SHAPES OF MOLECULES

Shapes of molecules

If we can draw Lewis structures for covalent molecules, we can predict their shapes

by applying a few simple rules. The theory that accounts for the shapes of molecules

is called valence shell electron pair repulsion theory(VSEPR theory) and is based on

the reasoning that electron pairs attempt to get as far away from other electron pairs

as possible because their negative charges repel each other.

Molecules with bonding electron pairs only

1. Consider the molecule BeCl 2

This can be represented as

There are two shared electron pairs (bonding pairs) around the central beryllium

atom. These two electron pairs arrange themselves as far apart from each other as they

can, so that the angle between the two Be–Cl bonds is 180° and the molecule is linear.

2. The BF 3 molecule

Now let us apply this theory to a situation where there are three pairs of bonding

electrons around the central atom, as in BF 3 :

5.2

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Example 5.1

Phosphorus forms two fluorides PF 3 and PF 5 :

In PF 3 , the octet rule is obeyed:

But in PF 5 , the central phosphorus atom has 10 electrons surrounding it:

Here, phosphorus uses its empty 3d orbitals to accommodate the extra electrons.

Covalently bonded

atoms with an

expanded octet

(i) Draw Lewis structures

for the following

molecules or ions:

(a)SF 6

(b)XeF 4

(c)BrF 3

(d)IF 5

(e)PCl 6 

(f)ClF 4 .

(ii)NF 3 exists, but NF 5

does not – suggest

why.

(iii)IF 7 exists, but ClF 7

does not – suggest a

reason for this.

Exercise 5C