what is electron gain enthalpy?

Electron gain enthalpy is the amount of energy released or absorbed when an isolated gaseous atom gains an electron to form a negative ion (anion).

Reaction:
X(g) + e⁻ → X⁻(g)

If energy is released, the electron gain enthalpy is negative. If energy is absorbed, it is positive.

Example:
Chlorine releases energy when it gains an electron, so it has a highly negative electron gain enthalpy.

In simple terms: Electron gain enthalpy measures the tendency of an atom to accept an electron. The more negative the value, the greater the atom's tendency to gain an electron.