Which of the following elements is not a Noble gas?
- (a)Radon
- (b)Krypton
- (c)Argon
- (d)Actinium
Correct — D, Actinium. Actinium (symbol Ac, atomic number 89) is a radioactive metal that heads the actinide series in Group 3 of the periodic table; it is not a noble gas. Radon, Krypton and Argon all belong to Group 18 — the noble (inert) gases with full outer electron shells. So the odd one out is Actinium.
- (a)Radon — Radon (Rn, atomic number 86) is a Group 18 noble gas — a radioactive one, but still a noble gas, so it is not the exception.
- (b)Krypton — Krypton (Kr, atomic number 36) is a Group 18 noble gas.
- (c)Argon — Argon (Ar, atomic number 18) is a Group 18 noble gas — in fact the most abundant noble gas in the Earth's atmosphere (~0.93%).
The noble (inert) gases are the Group 18 elements — helium, neon, argon, krypton, xenon and radon — whose complete valence shells make them largely unreactive. Actinium is not in Group 18; it is a Group 3 actinide, a reactive radioactive metal, so it does not belong to the noble-gas family.
The trap is that Radon is radioactive, like Actinium, which might tempt a candidate to exclude it. But radioactivity has nothing to do with being 'noble' — radon is still a Group 18 noble gas. The only non-noble-gas here is the metal Actinium.
- Group 18 (noble/inert) gases: He, Ne, Ar, Kr, Xe, Rn.
- Argon is the most abundant noble gas in the atmosphere (~0.93%).
- Radon is a radioactive noble gas (a decay product in the uranium/radium series).
- Actinium is a radioactive metal — the first element of the actinide series (Group 3), not a noble gas.
Rn, Kr and Ar are Group 18 noble gases; Actinium is a Group-3 actinide metal — the answer.
- Excluding Radon because it is radioactive — radioactivity does not make it non-noble
- Mistaking the unfamiliar name 'Actinium' for a gas
UPSC/UPPSC test periodic-table group membership through 'which is / is not a noble (inert) gas' or 'odd one out' phrasing.
Which one of the following is also called Stranger Gas?
- (a) Argon
- (b) Neon
- (c) Xenon
- (d) Nitrous oxide
Answer(c) Xenon
Same concept family — identifying noble (Group 18) gases; the odd option out (nitrous oxide) is the non-noble one, just as Actinium is here.
- practice — not a real PYQ
Which noble gas is the most abundant in the Earth's atmosphere?
- (a)Helium
- (b)Neon
- (c)Argon
- (d)Xenon
Answer(c) Argon — it makes up about 0.93% of air.
- practice — not a real PYQ
Actinium is the first element of which series in the periodic table?
- (a)Lanthanide series
- (b)Actinide series
- (c)Halogen group
- (d)Noble gas group
Answer(b) Actinide series — Actinium (Z = 89) heads the actinides.