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

Explanation

Out of O and F, F have higher negative electron gain enthalpy, because after the acceptance of one electron in F, it will acquire the stable noble gas configuration, whereas it is not so in case of O.
Out of P and Cl, Cl have higher negative electroni gain enthalpy. Because due to small size of F, the added electron experiences high repulsion. Hence, it is not stable and has low value of. electron gain enthaply and the size of Cl is large. So, interelectronic repulsion between the electrons is small, so the added electron experiences less repulsion and is stable and has high electron gain enthalpy. becomes positive.