Which of the following best explains why electron affinity values for the noble gases are endothermic?

• A. Noble gases have unpaired electrons in their outermost shell, which makes electron addition exothermic.
• B. Noble gases have low atomic radii, making it easier for them to attract additional electrons.
• C. Noble gases have completely filled valence shells, so adding an electron requires energy to overcome the stability of the closed shell.
• D. Noble gases have high electronegativity, so they readily accept electrons and release energy.

Answer: (C)

Verified step by step guidance

1. Understand that electron affinity refers to the energy change when an atom gains an electron. A negative value means energy is released (exothermic), while a positive value means energy is required (endothermic).
2. Recall that noble gases have completely filled valence electron shells, which makes them very stable and less likely to gain additional electrons.
3. Recognize that adding an electron to a noble gas means placing it into a new, higher energy orbital, which disrupts the stable, closed-shell configuration.
4. Because this process requires energy input to overcome the stability of the filled shell, the electron affinity values for noble gases are positive (endothermic).
5. Evaluate the given options and identify that the correct explanation is that noble gases have completely filled valence shells, so adding an electron requires energy to overcome the stability of the closed shell.