Chemistry, Third edition

76 5 · MORE ABOUT BONDING

ions in ionic substances. The melting and boiling points of substances which consist

of molecules with dipoles, therefore, tend to be much lower than those of ionic

solids, but higher than those substances that consist of non-polar molecules of

similar size.

London dispersion forces

London dispersion forces are another type of van der Waals’ force. They are attract-

ive forces between allmolecules (or atoms). How do we know that attractive forces

exist between non-polar molecules or neutral atoms? We know these forces exist

because elements and compounds that contain such particles can be liquefied and/or

solidified and this is only possible if attractive forces exist between particles in the

liquid and solid states. Iodine, for example, is a solid at room temperature and it

consists of I 2 molecules with no dipole moment. Similarly, although the smallest

particles of the inert or noble gases are neutral atoms, they can be liquefied.

How do these attractive London forces arise? Although non-polar molecules and

inert gas atoms have no dipole moment, the electron cloud in these particles, at one

instant, may be denser on one side of the particle than the other. This causes a tem-

porary dipole. The particle has, for a brief moment, one end with a small positive

charge and the other end with an equally small negative charge. This temporary

dipole causes (induces) another temporary dipole in a nearby particle – the positive

end of the first particle attracts electron density from a neighbouring particle, making

one end of the second particle slightly electron deficient and the other end electron

rich. This process of particles inducing dipoles in nearby particles continues

throughout. Weak attractive forces between the particles in the substance exist, at any

particular instant. As the electron density in molecules or atoms shifts around, these

dipoles change direction (fluctuate) but they are always present (Fig. 5.13).

The attractions due to London dispersion forces increase as the molecules or

atoms get larger. The number of electrons increases and the fluctuations in the elec-

tron density produce bigger temporary dipoles. Molecular shape also influences the

strength of the London interactions.

London dispersion forces are responsible for attractions between all types of

molecules, both polar and non-polar, and can range from weak to quite strong.

δ+ δ–

Temporar y

dipole

δ+ δ–

Induced

dipole

Fig. 5.13Temporary dipole-
induced dipole attractions.

BOX 5.4

Arrangement of molecules in

solid iodine

Covalent compounds can form crystals,

just like ionic substances. In black,

shiny crystals of iodine, the iodine

molecules (I 2 ) are arranged in a lattice

structure but they are held together by

London dispersion forces. Because

these forces are weak the iodine is a

volatile substance; the crystal structure

can be pulled apart very easily. The

arrangement is shown in Fig. 5.12.

Fig. 5.12The unit cell in solid iodine. The pairs of

overlapping circles indicate the orientations of the

I 2 molecules.