In which of the following pairs, electron gain enthalpies of constituent elements are nearly the same or identical?
(A) Rb and Cs
(B) Na and K
(C) Ar and Kr
(D) I and At
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Put the NCERT electron gain enthalpy values side by side and check which pairs almost match.

1. The general idea. Going down a group, the atom gets bigger, so an added electron is held less tightly. Lower in a group the size grows only slowly, so neighbouring values come out very close.

   Pair	Δ_(eg)H values (kJ mol⁻¹)	Difference	Nearly the same?
   (A) Rb and Cs	-47 and -46	1	Yes
   (B) Na and K	-53 and -48	5	No
   (C) Ar and Kr	+96 and +96	0	Yes, identical
   (D) I and At	-295 and -270	25	No

2. Noble gases. Ar and Kr have completely filled shells, so the extra electron must go into a new, higher shell. Both resist it equally, which gives the same positive value of +96 kJ mol⁻¹.
3. So. Only (A) Rb and Cs and (C) Ar and Kr have nearly equal or identical values. The answer is option (c).

The idea to keep. For alkali metals and noble gases, electron gain enthalpy hardly changes lower down the group. Learn the NCERT table values, because these questions turn on small differences.

Answer from the NTA final answer key. Solution by PrepWiser.