(i) Why is theh electron-gain enthalpy of oxygen is less than that of sulphur?
(ii) Arrange the following metal oxides in terms of ascending order of basicity: ZnO, MgO, CaO, CuO.

Explanation

(i) Electron-gain enthalpy of O is less than that S as the 2p-orbital of the outermost shell of O-atom is much smaller in size than the 3p-orbital of the outermost shell of S-atom, so, the additional electron-electron repulsive force produced due to addition of one electron to 2p-orbital of oxygen atom from outside is more than the additional electron-electron repulsive force developed by the addition of one electron to the 3p-orbital of S-atom. so the electron gain enthalpy of O-atom becomes less than that of S-atom.
(ii) `CuO lt ZnO lt MgO lt CaO`.