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

a. Ionisation enthalpy is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom so as to covert it into a gasous caton is called ionisation enthalpy. The force with which an electron is attracted by the nucleus of an atom is appreciably affected by the presence of other atoms within its molecule or in the neighbourhood. Therefore, for the purpose of determination of ionisation enthalpy, it is essential that these interatomoic 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 ionisation enthalpy, therefore, the interatomic distances are further reduced by carrying out the measurement at a low pressure of the gaseous atom. It is because of these reasons, that the term isolated gaseous atom has been included in the definition of ionisation enthalpy.
b. Electron gain enthalpy is the energy released when an isolated gaseous atom in the ground state accept an extra electron to form the 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 state, 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 into gaseous anion after accepting an electron. Therefore, for comparison purposes, the electron gain enthalpies of gaseous atoms must be determined in their respective most stable state, i.e. ground state.