Arrange the following permanent gases present in the atmosphere in ascending order: 1. Helium 2. Hydrogen 3. Neon 4. Xenon Select the answer using the code given below:

Correct — C, 2, 4, 1, 3. This is the option carried here, and the first thing to say about it is that it is not actually right — the correct sequence is not among the four on offer. Measured abundances in dry air are xenon 0.09 parts per million, hydrogen 0.55, helium 5.24 and neon 18.182, so from least abundant to most abundant the order is xenon, hydrogen, helium, neon, which in the paper's numbering is 4, 2, 1, 3. No option prints that. Option (c) comes closest: it has helium and neon in the right last two places and differs from the true answer only by transposing xenon and hydrogen at the front, a single swap. Every other option is further away, and option (a) is the exact reverse of the right answer. An item with no correct choice is normally cancelled once a key is published, and that is the honest verdict here. What is worth carrying away is the abundance ladder itself, which is stable and checkable: neon is the most plentiful of the four by a wide margin, xenon the rarest, and helium sits an order of magnitude below neon.

• (a)3, 1, 2, 4 — Neon, helium, hydrogen, xenon — the exact descending order. This is the trap for a student who has the abundances perfectly right and reads 'ascending' as 'starting from the biggest'. It is the mirror image of the true answer rather than an approximation to it.
• (b)3, 1, 4, 2 — Neon, helium, xenon, hydrogen — descending order with the last two swapped. Neon in front is the giveaway: neon is the most abundant of the four, so it cannot open an ascending list.
• (d)2, 4, 3, 1 — Hydrogen, xenon, neon, helium. It gets the front pair no better than (c) does and then puts neon ahead of helium, which reverses the one comparison the other options get right.

Dry air is overwhelmingly nitrogen and oxygen, with argon a distant third. Everything else is measured in parts per million. Among the gases named here, neon is the most plentiful at about 18 parts per million, helium follows at about 5, hydrogen at about half a part per million, and xenon is the rarest at about 0.09. 'Permanent gases' is the label for those whose share of dry air stays effectively the same from place to place through the lower atmosphere; water vapour, ozone and carbon dioxide are the ones that vary.

Two separate things can go wrong on an ordering item like this, and only one of them is the candidate's fault. The candidate's job is to hold the ladder — neon above helium above hydrogen above xenon — and then to read the direction word carefully, since the paper's own option (a) is the perfect descending answer waiting for anyone who does not. What went wrong on the paper's side is that the ascending sequence, 4, 2, 1, 3, was never printed. Xenon is the rarest of the four and belongs at the head of any ascending list; every option here opens with either neon or hydrogen. The card names this rather than manufacturing a reason to prefer one of the four, because inventing a justification would teach a wrong ordering as though it were right. A useful sanity check for the future: argon at 0.934 per cent is by far the commonest noble gas in air, and it is nearly five hundred times more abundant than neon, the next one down.

• NOAA's dry-atmosphere table: nitrogen 78.084 per cent, oxygen 20.946 per cent, argon 0.934 per cent, carbon dioxide 0.042 per cent.
• The trace gases in parts per million: neon 18.182, helium 5.24, methane 1.92, krypton 1.14, hydrogen 0.55, xenon 0.09.
• Ascending order of the four gases named in this question is xenon, hydrogen, helium, neon — the paper's 4, 2, 1, 3.
• Argon is the most abundant noble gas in the atmosphere, ahead of neon, helium, krypton and xenon in that order.
• Water vapour is the great variable: up to about four per cent by volume in the warm wet tropics and under one per cent over deserts and the poles, which is why composition tables are quoted for dry air.

• Reading 'ascending' as descending. On this paper the exact descending order is printed as an option.
• Ranking the noble gases by atomic number instead of by abundance — argon, which is heavier than neon, is far more plentiful.
• Assuming every ordering item has a correct option. Checking your own sequence against the list before choosing is what exposes a defective item.

Nearly always as an ordering or ranking item on atmospheric composition, sometimes as a single-line question asking which gas is most or least abundant.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following is the most abundant noble gas in the earth's atmosphere?

  • (a)Helium
  • (b)Neon
  • (c)Argon
  • (d)Xenon

  Answer(c) Argon — it makes up about 0.934 per cent of dry air, far more than neon at 18 parts per million or helium at 5 parts per million.

• practice — not a real PYQ

  In tables of atmospheric composition, the figures are quoted for dry air because

  • (a)water vapour is not a gas
  • (b)the share of water vapour varies widely with place and time
  • (c)water vapour is confined to the stratosphere
  • (d)water vapour has no effect on radiation

  Answer(b) the share of water vapour varies widely with place and time — from about four per cent in the warm wet tropics to under one per cent over deserts and the poles, so a single figure would mislead.