CDS (II) 2020 General Knowledge · Question 111 of 120

Correct — C, tritium. Hydrogen has three isotopes, all with one proton and differing only in the number of neutrons: protium with none, deuterium with one and tritium with two. Protium and deuterium are stable; tritium is not. Its nucleus, with two neutrons to a single proton, is too neutron-heavy to hold together indefinitely, and it decays by beta-minus emission — a neutron turns into a proton with the release of an electron and an antineutrino — becoming helium-3. The half-life is about 12·3 years and the beta particle emitted is unusually feeble at around 18·6 keV, weak enough to be stopped by a sheet of paper or the outer layer of skin, which is why tritium can safely be sealed into self-luminous watch dials and gunsights. It is also a fuel for fusion, paired with deuterium in thermonuclear devices and in reactors such as ITER.

• (a)protium — Protium is ordinary hydrogen — one proton, no neutron — and makes up more than 99·98 per cent of natural hydrogen. It is perfectly stable and is in fact the simplest stable nucleus there is.
• (b)deuterium — The near-miss, because deuterium sounds exotic and is used in heavy water, which moderates neutrons in reactors such as India's pressurised heavy water plants. But deuterium is stable — one proton and one neutron, a bound and non-decaying nucleus.
• (d)hydronium — Not an isotope at all. The hydronium ion, H₃O⁺, is what a proton becomes when it attaches to a water molecule, and it is the species that makes an aqueous solution acidic. The option is testing whether a candidate can tell an isotope from an ion.

Isotopes are atoms of the same element — the same number of protons, so the same chemistry — with different numbers of neutrons and therefore different masses. Hydrogen is the only element whose isotopes have separate names: protium, deuterium and tritium. Whether an isotope is stable depends on the balance between protons and neutrons in the nucleus; when the balance is wrong the nucleus reorganises itself by emitting radiation, which is radioactivity.

The item is really two tests in one. The first is whether the candidate can separate an isotope from an ion, which knocks out hydronium immediately. The second is which of the three real isotopes is unstable, and the ordering is worth remembering because it is not arbitrary: as neutrons pile up against a single proton the nucleus becomes less stable, so protium and deuterium hold and tritium does not. Deuterium is the option that costs marks, because it is the one associated in most students' minds with nuclear reactors — through heavy water, D₂O, which India uses as moderator and coolant in its pressurised heavy water reactors. Being useful in a nuclear reactor is not the same as being radioactive.

• Hydrogen has three isotopes: protium with no neutron, deuterium with one and tritium with two.
• Only tritium is radioactive; protium and deuterium are stable.
• Tritium decays by beta-minus emission to helium-3, with a half-life of about 12·3 years and a beta energy of about 18·6 keV.
• Heavy water, D₂O, is built on deuterium and is the moderator in India's pressurised heavy water reactors.
• Hydronium, H₃O⁺, is an ion and not an isotope; it is the species responsible for acidity in aqueous solution.

• Picking deuterium because heavy water is associated with nuclear reactors; deuterium is stable.
• Treating hydronium as a hydrogen isotope rather than as an ion.
• Assuming isotopes differ chemically; they differ in mass and nuclear stability, not in chemical behaviour.

As this isotope-identification item, as a which-is-not-radioactive item across several elements, or through half-life and radioactive-dating calculations.

• practice — not a real PYQ

  The number of neutrons in a tritium nucleus is

  • (a)0
  • (b)1
  • (c)2
  • (d)3

  Answer(c) 2 — tritium has mass number 3 and atomic number 1, so it carries one proton and two neutrons.

• practice — not a real PYQ

  Heavy water used in nuclear reactors contains which isotope of hydrogen?

  • (a)Protium
  • (b)Deuterium
  • (c)Tritium
  • (d)All three in equal proportion

  Answer(b) Deuterium — heavy water is D₂O, and it serves as a moderator and coolant in pressurised heavy water reactors.