Q.Which of the following pairs of elements would have a more negative electron gain enthalpy?

The key idea is that electron gain enthalpy depends on atomic size and effective nuclear charge. A more negative value means a greater release of energy when an electron is added.

(i) O or F: Both are in the second period. Fluorine has a higher nuclear charge and a smaller atomic radius than oxygen, so it attracts an incoming electron more strongly. However, oxygen's electron gain enthalpy is actually less negative than expected due to electron-electron repulsion in its compact $2p$ subshell. Fluorine has a more negative electron gain enthalpy. …

What makes electron gain enthalpy more negative?

Electron gain enthalpy is the energy change when a gaseous atom accepts an electron. A more negative value means more energy is released, i.e. the atom accepts the electron more readily. Two opposing factors decide the value:

• Nuclear charge / effective attraction — a stronger pull on the incoming electron makes the process more exothermic (more negative).
• Size and electron–electron repulsion — in a very small atom the incoming electron enters a compact subshell already crowded with electrons, so repulsion opposes the attraction and makes the value less negative than expected.

(i) O or F

Oxygen (Group 16) and fluorine (Group 17) are neighbours in Period 2. Fluorine has the higher nuclear charge ($Z = 9$ vs $8$) and is slightly smaller, so it attracts the incoming electron more strongly. This nuclear-charge advantage outweighs the extra repulsion, so fluorine releases more energy.

• $\Delta_{eg}H(\text{O}) \approx -141\ \text{kJ mol}^{-1}$
• $\Delta_{eg}H(\text{F}) \approx -328\ \text{kJ mol}^{-1}$

Fluorine's value is more negative.

(ii) F or Cl …