What is the significance of the terms — ‘isolated gaseous atom’ and ‘ground state’ while defining the ionization enthalpy and electron gain enthalpy?
Hint : Requirements for comparison purposes.

Explanation

The force exerted by the nucleus· of an atom on the electrons present in the atom is appreciably affected by the presence of other atoms in the molecule and in the neighbourhood. Since the magnitude of this force decides the values of ionisation enthalpy and electron gain enthalpy, it is necessary to define them for isolated atoms. It is not possible to isolate a single atom. Since in the gaseous state atoms (or molecules) are widely separated, ionisation enthalpy and electron gain enthalpy are defined for gaseous atoms and it is assumed that they are isolated. Further, the ground state refers to the state of minimum energy, i.e., the most stable state. If the atom is in the excited state, it will possess a certain amount of energy and the values of ionisation enthalpy and electron gain enthalpy measured in this state will be different. Hence, it is necessary to consider a gaseous atom in the ground state while defining ionisation enthalpy and electron gain enthalpy.