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.

Explanation

(i) Ionization enthalpy is the minimum amount of the energy required to remove the most loosely bound electrons increases isolated gaseous atom so as to convert it 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 it molecule or in the neighbourhood. Therefore for the purpose of determination of the ionization enthalpy it is essential that these interatimic forces of attraction should be minimum . since in the gaseous state the atom 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 reduced by carrying out the measurement at a low pressure of the gaseous atom,. It is because of these resons that the term isolated gaseous atom has been included in the definition of ionization enthalpy.
(ii) Electron gain enthalpy is the energy released when an isolated gaseous atom in the ground state accepts an extra electron to form the gaseous negative ion.
The term isolated gaseous atom has laready 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 enethalpies of gaseous atom must be determined in their respective most stable state i.e., ground state.