Electronegativity is the property of an atom that causes the shared electron pair in a heteronuclear molecule to be displaced more towards itself. For instance, in HF, the shared electron pair is displaced more towards fluorine because its electronegativity is far greater than that of hydrogen.

In contrast, electron gain enthalpy ($\Delta_{\text{eg}}H$) is the enthalpy change when a gas phase atom in its ground state gains an electron. This electron gain process may be exothermic or endothermic.

Thus, while electronegativity describes the displacement of a shared electron pair in a covalent bond, electron gain enthalpy measures the enthalpy change when a neutral gas phase atom acquires an extra electron.