In which of two groups of elements have highly negative electron gain enthalpies and readily add one or two electrons respectively to attain the stable noble gas configuration?

Correct option is D

Correct Answer: D

Explanation

• Group 17 (Halogens): These elements have a valence shell configuration of $$ns^2 np^5$$​. They require only one electron to attain the stable noble gas configuration ($$ns^2 np^6$$​). Due to their high effective nuclear charge and small atomic size, they have the most negative electron gain enthalpies.
• Group 16 (Chalcogens): These elements have a valence shell configuration of  $$ns^2 np^4.$$​ They need to add two electrons to achieve a stable octet. While their electron gain enthalpy is less negative than halogens, it remains significantly high compared to other groups.
• Stability Drive: The process of adding electrons is highly exothermic for these groups because the resulting ions achieve a lower energy, highly stable electronic state.

Information Booster

• Periodic Trend: Electron gain enthalpy generally becomes more negative as you move from left to right across a period because the atomic radius decreases and the effective nuclear charge increases.
• The Chlorine Anomaly: Although Fluorine is more electronegative, Chlorine has the most negative electron gain enthalpy in the periodic table. This is because the small size of the Fluorine atom leads to strong interelectronic repulsions in its 2p subshell, making it slightly harder to add an extra electron compared to the 3p subshell of Chlorine.

Additional Knowledge

• Group 18 (Noble Gases): These elements already have a complete octet. Adding an electron would require it to enter a new, higher-energy principal quantum level, resulting in large positive electron gain enthalpies.
• Group 15 (Nitrogen Family): These have a stable, half-filled $np^3$ configuration. They are less inclined to take an extra electron compared to Groups 16 and 17, and their electron gain enthalpies are much less negative (Nitrogen's is even slightly positive).
• Group 14 (Carbon Family): These require four electrons to reach a noble gas configuration. Their electron gain enthalpies are relatively low because the electrostatic attraction for an incoming electron is much weaker than in the elements further to the right.