Which is a noble gas?

**The reasoning** Noble gases are elements in **Group 18** (the last column) of the periodic table. They have a full outer electron shell, which makes them extremely stable and unreactive — they rarely form compounds with other elements. The noble gases are: Helium (He), Neon (Ne), **Argon (Ar)**, Krypton (Kr), Xenon (Xe), and Radon (Rn). Argon is the third noble gas. It has 18 electrons with a complete outer shell (2, 8, 8 configuration), so it doesn't need to gain or lose electrons. That's why it's chemically inert. **Why the wrong options tempt you** - **Oxygen (O₂)** — Very common gas in air (~21%), so students think "common gas = noble gas." But oxygen is highly reactive, not noble! - **Hydrogen (H₂)** — Lightest element, also a gas. But it's extremely reactive (burns easily), opposite of noble. - **Chlorine (Cl₂)** — A gas at room temperature, but it's a halogen (Group 17), very reactive and dangerous. **Quick takeaway** Noble gases are the "loners" of chemistry — they sit in Group 18, have full electron shells, and refuse to react with almost anything; memorize: He, Ne, Ar, Kr, Xe, Rn.