explain the electron gain enthalpy?

Electron Gain Enthalpy is defined as the enthalpy change taking place when an isolated gaseous atom accepts an electron to form a monovalent gaseous anion.

Variation across a period − Tends to become more negative as we go from left to right across a period. Because going from left to right across a period, tendency of the element to accept the electron increases. And hence a large amount of energy is released or electron gain enthalpy becomes more negative. For example fluorine has more negative electron gain enthalpy as compared to boron present in same period.

Variation down a group − Becomes less negative on going down the group. On going down the group, element's tendency to accept an electron decreases. For example, iodine has less negative electron gain enthalpy than fluorine present in the same group.