Define :
(a) Enthalpy of atomization
(b) Enthalpy of vaporization

Explanation

(a) Enthalpy of atomization : The enthalpy change accompanying the dissociation of all the molecules in one mole of a gas phase substance into gaseous atom is called enthalpy of atomization. It is denoted by `Delta_("ato")H`.
(b) Enthalpy of vaporization : The enthalpy change that accompanies the vaporization of one mole of liquid without changing its temperature at constant pressure is called enthalpy of vaporization. It is denoted by `Delta_("vap")H`.