(i)From the given figure, we can analyse that the chlorine atom is at the centre of the cube, i.e. at equal distance from all the eight corners of cube where caesium atoms are placed. Thus, due to symmetry, the force due to all Cs atoms, on Cl atom will cancel out.
Hence , E = F / q'
(ii)Thus, net force on Cl atom at A would be given by
where, r = distance between Cl ion and Cs ion. Applying Pythagoras theorem, we get