What is the significance of the terms-'isolated gaseous atom' and 'ground state' while defining the ionisation enthalpy and electron gain enthalpy?
Hint: Requirements for comparison purposes.

Answer

(i) Ionization enthalpy is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in ground state, so as to convert in into a gaseous cation. The force with which an electron is attracted by the nucleus of an atom is appreciably affected by presence of other atoms within its molecule or in the neighbourhood. Therefore, for the purpose of determination of ionization enthalpy, it is essential that these interatomic forces of attraction should be minimum. Since in the gaseous state, the atoms are widely separated, therefore these interatomic forces are minimum. Further, since it is not possible to isolate a single atom for the purpose of determination of its ionization enthalpy, therefore the interatomic distances are further increased by carrying out the measurement of the gaseous atom at a low pressure. It is because of these reasons, that the term isolated gaseous atom has been included in the definition of ionization enthalpy.
(ii) Electron gain enthalpy is the enthalpy change when as isolated gasoeus atom in the ground state accepts an extra electron to form gaseous negative ion. The term isolated gaseous atom has already been explained above. The term ground state here means that the atom must be present in the most stable electronic configuration i.e., the ground state. The reason being that when the isolated gaseous atom is in the excited state, lesser amount of energy will be released when it gets converted inot gaseous anion after accepting an electron and lesser amount of energy will be required when it gets converted into gaseous cation. Therefore, for comparison purpose, the ionisation enthalpies and electron gain enthalpies of gaseous atoms must be determined in their respective most stabel state, i.e. ground state.