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4 LIQUIDS AND SOLIDS eLearn.Punjab

4.1.3 Instantaneous Dipole-induced Dipole Forces or London Dispersion Forces

Intermolecular forces among the polar molecules, as discussed in section 4.1.1 are very easy to
understand. But the forces of attraction present among the non-polar molecules like helium,
neon, argon, chlorine and methane need special attention because under normal conditions
such molecules don’t have dipoles. We know that helium gas can be liquefied under appropriate
conditions. In other words forces of attraction operate among the atoms of helium which cause
them to cling together in the liquid state.
A German physicist Fritz London in 1930 offered a simple explanation for these weak attractive
forces between non-polar molecules.
In helium gas, the electrons of one atom influence the moving electrons of the other atom.
Electrons repel each other and they tend to stay as far apart -as possible. When the electrons of
one atom come close to the electron of other atom, they are pushed away from each other. In this
way,a temporary dipole is created in the atom as shown in the Fig (4.3). The result is that, at any
moment, the electron density of the atom is no more symmetrical. It has more negative charge on
one side than on the other. At that particular instant, the helium atom becomes a dipole. This is
called instantaneous dipole.

Fig. (4.3) Instantaneous dipole-induced
dipole attractions between helium atoms.

This instantaneous dipole then disturbs the electronic cloud of the other nearby atom. So,a dipole
is induced in the second atom. This is called induced dipole. The momentary force of attraction
created between instantaneous dipole and the induced dipole is called instantaneous dipole-
induced dipole interaction or London force.

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