Q.Assertion (A): Electron gain enthalpy becomes less negative as we go down a group.
Reason (R): Size of the atom increases on going down the group and the added electron would be farther from the nucleus.

Concept: Electron gain enthalpy ($\Delta_{eg}H$) is the enthalpy change when an isolated gaseous atom accepts an electron; a more negative value means the addition is more favourable.

Assertion (A) claims $\Delta_{eg}H$ becomes less negative all the way down a group. This is wrong as stated: on descending from the 2nd to the 3rd period the value actually becomes MORE negative — chlorine ($-349$ kJ mol$^{-1}$) is more negative than fluorine ($-328$), and sulphur ($-200$) more negative than oxygen ($-141$). The compact $n = 2$ shell of F and O crowds the incoming electron with extra repulsion; in the roomier $n = 3$ shell of Cl and S that repulsion is smaller. Only from the t …

Electron gain enthalpy measures the energy change when an isolated gaseous atom accepts an electron to form an anion; the more negative the value, the more favourable the addition.

Step-by-step analysis

1. Test the assertion against real values (Group 17):
   • Fluorine: $-328$ kJ mol$^{-1}$
   • Chlorine: $-349$ kJ mol$^{-1}$
   • Bromine: $-325$ kJ mol$^{-1}$
   • Iodine: $-295$ kJ mol$^{-1}$ …