Under which condition is an atom generally least reactive?

• A. When it has a half-filled outermost shell
• B. When it has an unpaired electron in its outermost shell
• C. When its outermost electron shell is completely filled
• D. When it has one electron in its outermost shell

Answer: (C)

Verified step by step guidance

1. Understand that atomic reactivity is largely influenced by the electron configuration of the atom, especially the electrons in the outermost shell (valence electrons).
2. Recall that atoms tend to be most stable and least reactive when their outermost electron shell is completely filled, as this configuration corresponds to a low energy, stable state.
3. Recognize that atoms with half-filled outermost shells or unpaired electrons tend to be more reactive because they seek to gain, lose, or share electrons to achieve a full outer shell.
4. Note that atoms with only one electron in their outermost shell are typically highly reactive because they tend to lose that electron easily to achieve a stable configuration.
5. Conclude that the condition under which an atom is generally least reactive is when its outermost electron shell is completely filled, making it energetically stable and less likely to participate in chemical reactions.