Q.Define electron affinity.

Electron Gain Enthalpy

Electron gain enthalpy ($\Delta_{eg}H$) is the enthalpy change when a gaseous atom accepts an electron to form a gaseous anion: $\mathrm{X(g) + e^- \to X^-(g)}$. A large negative value means energy is released and the atom readily accepts an electron; a positive value (e.g. noble gases $\mathrm{Xe(g)\to Xe^-}$, and the stable configurations of Be and Mg with filled s-orbitals) means the process is endothermic.

Trends: across a period $\Delta_{eg}H$ generally becomes more negative (increasing nuclear charge, smaller size); down a group it becomes less negative. The most striking exception is that the second-period elements are less exothermic than their third-period congeners because their small, compact 2p orbitals cause strong electron–electron repulsion. Hence the order for halogens is $\mathrm{Cl}\ (-349) > \mathrm{F}\ (-328) > \mathrm{Br}\ (-325) > \mathrm{I}\ (-295)\ \mathrm{kJ\,mol^{-1}}$, and for the oxygen family $\mathrm{S > O}$. This makes chl …