Chemistry, Third edition

MOLECULES WITH AND WITHOUT DIPOLES 71

Molecules with and without dipoles

In order to work out whether a molecule has a dipole or not we need to consider

1.whether the molecule contains polar covalent bonds;

2.the shape of the molecule.

Molecules without dipoles

Carbon dioxide is a linear molecule, containing carbon and oxygen atoms:

O=C=O

The electronegativity difference between carbon (electronegativity 2.5) and oxygen

(electronegativity 3.5) is 1, so the C=O bond is polar:

C=O

The carbon dioxide molecule, however, does not have an overall dipolebecause it is

linear:

O=C=O

The dipoles of each polar C=O bond are equal in size but point in opposite directions

and cancel each other out. So, the molecule does not have an overall dipole –

although it contains polar bonds, the molecule is non-polar overall.

Carbon dioxide is an example of those molecules which, although they contain

polar bonds, do not have an overall dipole because the geometry of the molecules

allow the bond dipoles to cancel each other out. These geometries include trigonal

planar, tetrahedral, trigonal bipyramidal and octahedral. Note that for this to be

true,all the covalent bonds must be the same. Some examples are

Molecules with dipoles

A water molecule is V-shaped and, because of the difference in electronegativities

between oxygen and hydrogen; it contains polar covalent bonds:

Because the molecule is not linear, it has a positive end and a negative end – it has a

dipole and is polar overall, as shown in Fig. 5.3.

5.4

Negative

end

Direction

of dipole

+

Positive

end

Fig. 5.3A polar molecule.