Question

Identify the pairs in which electron gain enthalpy of the first element is more than that of the second element. \[ \text{I. } F,\ Br \] \[ \text{II. } Na,\ Li \] \[ \text{III. } S,\ O \]

• A. I, II, III
• B. I, III only
• C. II, III only
• D. I, II only

Hint:

Important exceptions: \[ Cl>F,\qquad S>O,\qquad Na>Li \] for electron gain enthalpy.

Answer 1

The Correct Option is C

Concept: Electron gain enthalpy generally becomes more negative across a period and less negative down a group. However, there are important exceptions.

Step 1: Examine Pair I : \(F,Br\).
Electron gain enthalpy values: \[ Cl>F. \] Also, \[ F>Br. \] Since fluorine has a more negative electron gain enthalpy than bromine, Pair I is correct.

Step 2: Examine Pair II : \(Na,Li\).
Lithium is very small in size. Due to stronger electron-electron repulsion in the compact \(2s\) orbital, electron gain enthalpy of sodium is slightly more negative. Hence, \[ Na>Li. \] Pair II is correct.

Step 3: Examine Pair III : \(S,O\).
Oxygen is exceptionally small. Incoming electron experiences strong repulsion. Therefore, \[ S>O. \] Pair III is also correct. Thus, \[ \boxed{\text{II and III only}} \]