Ionization enthalpy is the energy required to remove an electron from an isolated gaseous

atom in its ground state. Although the atoms are widely separated in the gaseous state,

there are some amounts of attractive forces among the atoms. To determine the ionization

enthalpy, it is impossible to isolate a single atom. But, the force of attraction can be further

reduced by lowering the pressure. For this reason, the term ‘isolated gaseous atom’ is

used in the definition of ionization enthalpy.

Ground state of an atom refers to the most stable state of an atom. If an isolated gaseous

atom is in its ground state, then less amount energy would be required to remove an

electron from it. Therefore, for comparison purposes, ionization enthalpy and electron gain

enthalpy must be determined for an ‘isolated gaseous atom’ and its ‘ground state’.