Which of the following elements has the greatest electron affinity (largest positive value)?

• A. Na
• B. Cl
• C. O
• D. Ar

Answer: (B)

Verified step by step guidance

1. Understand that electron affinity is the energy change that occurs when an atom gains an electron. A more negative electron affinity value means the atom releases more energy and has a stronger tendency to gain an electron.
2. Recall the general trend of electron affinity in the periodic table: it increases (becomes more negative) across a period from left to right and decreases down a group.
3. Identify the positions of the elements in the periodic table: Na (Group 1, Period 3), O (Group 16, Period 2), Cl (Group 17, Period 3), and Ar (Group 18, Period 3).
4. Compare the elements based on their group and period: halogens (Group 17) like Cl typically have the highest electron affinities because they are one electron short of a full octet, making them very eager to gain an electron.
5. Conclude that Cl has the greatest electron affinity among the given elements because it is a halogen with a nearly full valence shell, leading to a large release of energy when it gains an electron.