Question

Define electron gain enthalpy.

Answer 1

It relates to the gain of electron(s) by a neutral atom that then becomes an anion. It is fundamentally opposite to ionization potential.

Complete step-by-step answer:
We often come across compounds that are electrovalent in nature. Electrovalent compounds are very different from the covalent compounds, as the former are products of exchange of electrons between electron donating and withdrawing species while the latter is a product of mutual sharing of electrons among two or more atomic species.
As exchange of electrons is how electrovalent compounds (also known as ionic compounds) form, two types of species take part in this process, namely the cation and anion. The atomic species which are good at forming anion show the phenomenon of gaining electrons. The enthalpy change that is associated with this process is termed as electron-gain enthalpy. The accepted definition of electron gain enthalpy is as follows:

The enthalpy change associated with the gain of an electron by a neutral isolated gaseous atom in its ground state, is termed as electron gain enthalpy.

This enthalpy can be negative or positive; which also means that the process of gaining an electron can be either exothermic (when energy is released into the surrounding from the system) or endothermic (when energy is absorbed by the system from the surrounding) respectively. This is unlike ionisation potential which is always positive because in that case, energy has to be provided to an atom in order to extract an electron from its valence shell.

Note: The electron gain enthalpy of non-metals is highly negative and that of the metals is highly positive. So, a stable ionic molecule is formed when an anion with negative electron gain enthalpy is paired with an atom with low ionisation enthalpy.