Discuss the factors affecting electron gain enthalpy and the trend in its variation in the periodic table.

(N/A) When an electron is added to the valence shell of an isolated gaseous atom,the electron gain enthalpy of an element is equal to the energy released. $A_{(g)} + e^{-} \longrightarrow A^{-}_{(g)}$; $\Delta_{eg}H = \text{negative}$.
Factors affecting electron gain enthalpy:
$(i)$ Effective nuclear charge: As the attraction of the nucleus towards the incoming electron increases,the electron gain enthalpy becomes more negative (increases in magnitude) with an increase in effective nuclear charge.
$(ii)$ Size of an atom: As the atomic size increases,the electron gain enthalpy becomes less negative because the incoming electron is farther from the nucleus.
$(iii)$ Type of subshell: The ease of adding an electron depends on the subshell proximity. The general order of stability for adding an electron is $s > p > d > f$.
$(iv)$ Electronic configuration: Half-filled and fully-filled subshells have stable configurations,making the addition of an electron energetically unfavorable.
Variation in the periodic table:
Across a period: Generally,electron gain enthalpy becomes more negative from left to right due to an increase in effective nuclear charge and a decrease in atomic size.
Down a group: Electron gain enthalpy becomes less negative from top to bottom because the atomic size increases,placing the added electron farther from the nucleus.
Note: Electron gain enthalpy of $O$ or $F$ is less negative than that of the successive elements ($S$ or $Cl$) because the added electron in $n=2$ experiences significant inter-electronic repulsion compared to the larger $n=3$ shell.