Which of the following pairs of elements



Which of the following pairs of elements would have a more negative electron gain enthalpy?
(i)O or F (ii) F or Cl


(i) Oxygen and fluorine both belong to second period. Electron gain enthalpy generally becomes more negative across a period as we move from left to right. On moving from oxygen to fluorine, the effective nuclear charge increases and atomic size decreases with increase in atomic number. Due to this, forces of attraction of the nucleus increases for the incoming electron. That’s why electron gain enthalpy becomes more negative for fluorine than that of oxygen. Furthermore fluorine attain stable gas configuration by picking up an electron.

Therefore, electron gain enthalpy of fluorine is much more negative ( -328 kj ${{mol}^{-1}}$) than that of oxygen (-141 kj ${{mol}^{-1}}$).
(ii) Within a group, electron gain enthalpy becomes less negative down a group. But electron gain enthalpy of chlorine is more negative (- 349 kj ${{mol}^{-1}}$) than that of the fluorine (- 328 kj ${{mol}^{-1}}$).
This is due to small size of fluorine as the electron-electron repulsions in relatively compact 2p-orbital is greater than that in the larger 3p-orbital and hence, the incoming electron feels greater repulsion in fluorine than in the chlorine. That’s why chlorine have more negative electron gain enthalpy than that of fluorine.