Which of the following is a noble gas?

• A. Argon
• B. Oxygen
• C. Nitrogen
• D. Fluorine

Answer: (D)

Noble gases have a complete outer electron shell, which makes them very unreactive and usually present as monatomic gases. Argon fits this pattern: it’s in Group 18 and ends with a full 8-electron valence shell (argon’s outer configuration is complete), so it hardly forms compounds and remains a quiet, inert gas under ordinary conditions. In contrast, oxygen and fluorine are highly reactive nonmetals—they actively seek to complete their octets, either by bonding or by gaining electrons, which is why they’re not noble gases. Nitrogen is a nonmetal as well, and while N2 is a very strong, stable diatomic molecule, nitrogen itself isn’t a noble gas. Hence, Argon is the noble gas in the list.