For which of the following elements is the gaining of an electron most exothermic?

• A. O
• B. Na
• C. Cl
• D. Ar

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 a more exothermic electron affinity means the atom releases more energy upon gaining an electron, indicating a stronger tendency to accept an electron.
3. Consider the position of the elements in the periodic table: Oxygen (O) and Chlorine (Cl) are nonmetals, Sodium (Na) is an alkali metal, and Argon (Ar) is a noble gas.
4. Recognize that noble gases like Argon have very stable electron configurations and generally have positive or very low electron affinities, meaning they do not release energy when gaining an electron.
5. Compare the electron affinities of the given elements, noting that halogens like Chlorine typically have the most exothermic electron affinities because they are one electron short of a full octet, making the addition of an electron highly favorable.