Question

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 :

• (A) and (B) only
• (B) and (C) only
• (A) and (C) only
• (C) and (D) only

Answer 1

The Correct Option is C

To determine which pairs of elements have nearly the same or identical electron gain enthalpies, we must understand the concept of electron gain enthalpy. It refers to the energy change when an electron is added to a neutral atom in the gaseous state to form a negatively charged ion. In simple terms, it's a measure of how much energy is needed for an atom to gain an electron.

Now, let's analyze the given pairs:

1. Option (A) - $Rb$ and $Cs$: Both Rubidium (Rb) and Cesium (Cs) belong to the same group (alkali metals) in the periodic table, and they generally show low electron gain enthalpies due to their large atomic size and low electronegativity. Therefore, the electron gain enthalpies of Rb and Cs are indeed nearly the same.
2. Option (B) - $Na$ and $K$: Sodium (Na) and Potassium (K) are also alkali metals. However, their electron gain enthalpies are not nearly the same as their atomic sizes and effective nuclear charges differ significantly.
3. Option (C) - $Ar$ and $Kr$: Argon (Ar) and Krypton (Kr) are noble gases. The addition of an electron to a noble gas is energetically unfavorable, making their electron gain enthalpies close to zero. Hence, Ar and Kr have nearly identical electron gain enthalpies.
4. Option (D) - $I$ and $At$: Iodine (I) and Astatine (At) are halogens. The electron gain enthalpies for halogens tend to decrease down the group; hence, they are not nearly the same for I and At.

Based on this analysis, we find that the pairs whose electron gain enthalpies are nearly the same or identical are: (A) $Rb$ and $Cs$ and (C) $Ar$ and $Kr$.

Therefore, the correct answer is: (A) and (C) only.