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

We need to identify which pairs of elements have nearly the same or identical electron gain enthalpies.

Pair (A): Rb and Cs

Both rubidium and caesium are alkali metals in the same group. As we go down the group, the electron gain enthalpy becomes less negative but for heavier alkali metals (Rb and Cs), the values become very similar due to the shielding effect and large atomic size. The electron gain enthalpies of Rb and Cs are nearly identical (both around -47 to -46 kJ/mol).

Pair (B): Na and K

Sodium and potassium are also alkali metals, but their electron gain enthalpies differ noticeably. Na has an electron gain enthalpy of about -53 kJ/mol while K has about -48 kJ/mol. These are not nearly the same.

Pair (C): Ar and Kr

Both argon and krypton are noble gases with completely filled electron shells. Noble gases have near-zero (positive) electron gain enthalpies because they resist gaining an extra electron. The electron gain enthalpies of Ar and Kr are nearly identical (both are positive and very close to zero).

Pair (D): I and At

Iodine and astatine are halogens. Their electron gain enthalpies are different — iodine has a much more negative value compared to astatine, which is a larger and more metallic element.

Therefore, the pairs with nearly identical electron gain enthalpies are A (Rb and Cs) and C (Ar and Kr).

The correct answer is Option C: A & C only.