Which of the following elements has the highest electron affinity?

• A. Na
• B. O
• C. F
• D. Cl

Answer: (D)

Verified step by step guidance

1. Understand that electron affinity is the energy change that occurs when an atom gains an electron, typically measured in kJ/mol. A higher (more negative) electron affinity means the atom more readily accepts an electron.
2. Recall the general periodic trend: electron affinity tends to increase (become more negative) across a period from left to right and decrease down a group in the periodic table.
3. Compare the given elements: Na (sodium) is in Group 1, Period 3; O (oxygen) is in Group 16, Period 2; F (fluorine) is in Group 17, Period 2; and Cl (chlorine) is in Group 17, Period 3.
4. Recognize that halogens (Group 17) have high electron affinities because they need only one electron to complete their valence shell, making them very eager to gain an electron.
5. Note that although fluorine is higher in the group, chlorine actually has a higher electron affinity due to less electron-electron repulsion in its larger 3p orbital compared to fluorine's smaller 2p orbital, which explains why Cl has the highest electron affinity among the options.