Q.Which of the following elements will gain one electron more readily in comparison to other elements of their group? (Note: more than one of the given options may be correct.)

The key idea is electron gain enthalpy under standard state conditions (gaseous atoms). A more negative value means the element gains an electron more readily.

• In Group 16, oxygen has a less negative electron gain enthalpy than sulfur due to its small size and high electron-electron repulsion in the compact $2p$ subshell. So S (g) gains an electron more readily than O (g). …

The question asks which elements will gain one electron more readily compared to other elements of their group. "More readily" means a more negative electron gain enthalpy — the atom releases more energy when it accepts an electron, making the process energetically favourable. We compare each element with others in its own group, not across groups.

Let’s examine each option one by one.

1. Option (A): S (g) — compare with other Group 16 elements

   In Group 16, the elements are O, S, Se, Te, Po. Electron gain enthalpy becomes less negative as we go down the group because atomic size increases and the added electron feels less nuclear attraction.

   Oxygen has a less negative electron gain enthalpy than sulphur due to its small size causing electron-electron repulsion in the compact 2p subshell. But the question asks which gains more readily — that would be the one with the most negative value.

   Among Group 16, oxygen actually has a less negative value than sulphur, so S is not the most ready electron gainer in its group. In fact, S gains an electron more readily than O, but the question compares "in comparison to other elements of their group" — meaning we check if S is the most ready. It is not; oxygen is less ready, but selenium, tellurium are even less ready. So S is not the standout.

   Watch out

   A common mistake is to think oxygen gains an electron most readily because it is the smallest. But the small size of oxygen leads to high electron-electron repulsion in its 2p orbital, making its electron gain enthalpy less negative than sulphur's. So S is actually more ready than O, but not the most ready in its group — the trend is irregular.

2. Option (B): Na (g) — compare with other Group 1 elements

   Group 1 elements (alkali metals) have very low electron gain enthalpies (close to zero or even positive) because they already have a stable ns¹ configuration and adding an electron disrupts that. As we go down the group, electron gain enthalpy becomes slightly more negative (less unfavourable) because the added electron goes into a larger orbital with less repulsion.

   Sodium is at the top of the group (after Li). Its electron gain enthalpy is actually less negative than that of potassium, rubidium, etc. So Na does not gain an electron more readily than other Group 1 elements. In fact, it gains an electron less readily.

   So (B) is not correct.

3. Option (C): O (g) — compare with other Group 16 elements

   As discussed, oxygen has a less negative electron gain enthalpy than sulphur due to small size and electron repulsion. So oxygen does not gain an electron more readily than other Group 16 elements. In fact, it gains less readily than sulphur.

   Tip

   The anomaly in Group 16: O has electron gain enthalpy = -141 kJ/mol, while S has -200 kJ/mol. So S gains an electron more readily than O. This is a classic exception to the general trend. …