CoachThe polarisability of an atom, ion or molecule measures how easily its electron cloud is deformed by an electric field — the field of a neighbouring ion or dipole. A polarisable species acquires, in a field, an induced dipole moment proportional to the field.
Examples
Example 2.21 (Why polarisability matters)
The London attraction between molecules, the subject of Chapter 4, grows with polarisability: diiodine is a solid at room temperature, dichlorine a gas. In an ionic crystal, a small highly charged cation polarises a large anion and gives the bond some covalent character, the failure of the simple ionic model met in Chapter 6.