For which of the following elements will gaining an electron be the most exothermic process?

• A. Na
• B. Ar
• C. Cl
• D. O

Answer: (C)

Verified step by step guidance

1. Understand that the process of gaining an electron corresponds to the electron affinity of an element, which is the energy change when an atom in the gas phase gains an electron to form an anion.
2. Recall that elements with nearly full valence shells tend to have more exothermic (more negative) electron affinities because gaining an electron completes or nearly completes their octet, releasing energy.
3. Consider the given elements: Na (a group 1 metal), Ar (a noble gas), Cl (a halogen), and O (a chalcogen). Among these, halogens like Cl have high electron affinities because they need only one electron to complete their valence shell.
4. Recognize that noble gases like Ar have very low or positive electron affinities because their valence shells are already full, so gaining an electron is not favorable.
5. Conclude that Cl will have the most exothermic electron affinity among the options because it gains an electron to achieve a stable octet, releasing more energy compared to Na, Ar, or O.