Define electron gain enthalpy. How it varies in a group and in a period ? Why is the electron gain enthalpy of O or F is less negative than that of succeeding element in the group ?

Explanation

(1) Electron affinity is the amount of energy released when an electron is added to a neutral gaseous atom in its ground state. It is known as first Electron affinity `EA_(1)`. It has -ve value. `X_((g)) + e rarr X_((g))^(-) + EA_(1)`
(2) Energy can be absorbed when an other electron is added to uninegative ion. It is because to overcome the repulsion between negative ion and electron. Hence second electron affinity, `EA_(2)` has +ve value, `X_((g))^(-) + e rarr X_((g))^(2-) + EA_(2)`
(3) It can be indirectly determined from Born- Haber cycle. Its units are kJ/mole
4. Trend in a group: Generally EA decreases down the group. In a group the second element has higher EA value than the first member. e.g: EA of Cl is more than that of F
Fluorine possess lower EA value than chlorine. It is due to inner electron repulsions in Fluorine.
5. Trend in a period: Generally EA value increases across a period, but some irregularities can be observed.
(a) IA group elements possess low EA values than the corresponding IIIA elements. e.g: Be has EA value zero, it is due to completely filled `2s^(2)` orbital.
(b) VA elements have low EA values than that of VI A elements. e.g: Due to presence of half filled p-orbitals `(2s^(2)2p^(3))`, nitrogen has lower EA value than that of oxygen.
(c ) For inert gases EA value is zero.
(d) The element with the highest EA value is chlorine.
`rarr` The electron gain enthalpy of O and F is less negative than the succeeding element in the group because. These have small size and inter electronic repulsions are high in these elements
`O rarr - 141` KJ/mole and `S rarr -200` kJ/mole
`F rarr -3280` KJ/mole and `Cl rarr -349` kJ/mole