Which of the following elements releases the most energy when gaining an electron?

• A. O
• B. Na
• C. Cl
• D. Ar

Answer: (C)

Verified step by step guidance

1. Understand that the energy released when an atom gains an electron is called the electron affinity. It generally measures how much an atom wants to gain an electron.
2. Recall that electron affinity tends to increase across a period (left to right) in the periodic table because atoms become more eager to fill their valence shells.
3. Note that halogens (Group 17 elements like Cl) have high electron affinities because gaining one electron completes their valence shell, making them very stable.
4. Compare the given elements: O (Group 16), Na (Group 1), Cl (Group 17), and Ar (Group 18). Ar is a noble gas with a full valence shell and typically has very low or positive electron affinity (does not release energy).
5. Conclude that among these, Cl releases the most energy when gaining an electron due to its high electron affinity as a halogen, which aligns with the periodic trends.