Modern inorganic chemistry

38 STRUCTURE AND BONDING

Table 2.8

SHAPES OF MOLECULES AND IONS

Electron pairs Essential shape

1 linear

2 linear

3 trigonal planar

4 tetrahedral

5 trigonal bipyramidal

6 octahedral

Methane, CH 4 , for example, has a central carbon atom bonded to

four hydrogen atoms and the shape is a regular tetrahedron with a

H— C— H bond angle of 109°28', exactly that calculated. Electrons

in a lone pair', a pair of electrons not used in bonding, occupy a

larger fraction of space adjacent to their parent atom since they are

under the influence of one nucleus, unlike bonding pairs of electrons

which are under the influence of two nuclei. Thus, whenever a lone

pair is present some distortion of the essential shape occurs.

Consider ammonia, NH 3 :

H : N : H i.e. H— N— H

3H<

H H

In this case we have three bonding pairs and one lone pair. The

essential shape is, therefore, tetrahedral but this is distorted due to

the presence of the lone pair of electrons, the H — N — H bond angle

bein 107°:

[ ; [ \Regionoccupied

w \ I by lone pair

When the ammonium ion NH^ is formed the lone pair becomes a
bonding pair and the shape becomes a regular tetrahedron.
The distortion due to the presence of lone pairs of electrons is
more marked in water :

xb*xx H$OXX i.e. H — Qx

~~ « i

+ 2 H-