CDS (II) 2023 General Knowledge · Question 42 of 120

Correct — B, 94. The International Union of Pure and Applied Chemistry recognises 118 chemical elements, and the division between them is drawn at atomic number 94: the first 94 occur naturally on Earth, and the remaining 24 are synthetic, made in nuclear reactions in reactors and accelerators. So the count of naturally occurring elements is 94, running from hydrogen at atomic number 1 to plutonium at 94. Six members of that list are present only in vanishingly small traces — technetium, promethium, astatine, francium, neptunium and plutonium — which is why an older and still widely repeated count is 92, counting only hydrogen through uranium, the elements found in appreciable quantity. Both numbers describe something real, but the modern count of elements that occur naturally, including the trace occurrences, is 94, and that is the number the key takes.

• (a)114 — Too high, and not a count of natural elements at all. The recognised total of elements, natural and synthetic together, is 118, and everything above atomic number 94 is made artificially. No count of naturally occurring elements reaches into the hundreds.
• (c)86 — This is the atomic number of radon, not a count of anything. It appears plausible only because it sits between the familiar 82 and 92, and there is no classification of the elements that stops at 86.
• (d)82 — Again an atomic number rather than a count — 82 is lead. The genuinely competing figure in this question is 92, hydrogen to uranium, which is not offered; 82 is simply lower than any defensible answer.

The elements are ordered by atomic number, the count of protons a nucleus holds, and the sequence runs complete and unbroken from 1 to 118. Where an element sits in that sequence decides whether it can survive on Earth at all. Light and middleweight nuclei are stable or nearly so, and are found in rock, water and air. Beyond a point the nuclei become so unstable that any quantity present when the Earth formed has long since decayed, and those elements exist today only because physicists build them, atom by atom, by firing nuclei at heavy targets.

The item is pure recall, but it can be reconstructed from two anchors. The total is 118, which is the number the periodic table has stood at since 2016 when the last four were named. The natural-synthetic boundary is at plutonium, atomic number 94, the last element found in nature — in trace amounts in uranium ores — rather than only in a laboratory. Subtracting gives 24 synthetic elements. The reason candidates hesitate is the rival figure of 92, which counts only up to uranium; a candidate who remembers 92 and does not find it among the options should reach for 94 rather than for a much lower number, because the two are the only counts in serious use. Nothing here has changed since the 2023 exam, as no new element has been named since 2016.

• IUPAC recognises 118 chemical elements in all.
• 'The first 94 occur naturally on Earth, and the remaining 24 are synthetic elements produced in nuclear reactions.'
• Six of the 94 occur only in extreme trace amounts: technetium (43), promethium (61), astatine (85), francium (87), neptunium (93) and plutonium (94).
• The older textbook figure of 92 counts hydrogen through uranium — the elements present in appreciable quantity — which is why students often pick it.
• 'Naturally occurring' in this statement means occurring on Earth; the last four elements were named in 2016, so the total has stood at 118 since then.

• Answering 92 out of habit. It is the count up to uranium, not the count of elements that occur naturally, and in this paper it is not even offered.
• Confusing an atomic number with a count. Both 86 and 82 are atomic numbers, of radon and lead.
• Assuming every naturally occurring element is abundant; six of the 94 exist only as traces in ores or as decay products.

As a direct number-recall item, or embedded in a statements question on the periodic table, the total count of elements, or which elements are synthetic.

• practice — not a real PYQ

  The total number of chemical elements recognised by IUPAC is

  • (a)92
  • (b)94
  • (c)112
  • (d)118

  Answer(d) 118 — of these the first 94 occur naturally on Earth and the remaining 24 are synthetic; the total has stood at 118 since the last four were named in 2016.

• practice — not a real PYQ

  Which one of the following is the heaviest element found in nature, though only in trace amounts?

  • (a)Uranium
  • (b)Neptunium
  • (c)Plutonium
  • (d)Americium

  Answer(c) Plutonium — atomic number 94, the last naturally occurring element; uranium at 92 is the heaviest found in appreciable quantity, and americium at 95 is purely synthetic.