An atom of atomic number Z can be modelled as a point positive charge surrounded by a rigid uniformly negatively charged solid sphere of radius R. The electric polarisability a of this system is defined as (a) where PE is the dipole moment induced on application of electric field E which is small compared to the binding electric field inside the atom. It follows that a = J ATEDR 8 R3 (b) = 4-TTED R3 PE E (c) 8ER