MPSC 2024 Prelims Paper-I · Question 69 of 100

Correct — option (2), 'Only (b)', deuterium. Hydrogen has three isotopes, all with a single proton and therefore all chemically hydrogen, differing only in the number of neutrons in the nucleus. Protium, written as hydrogen-1, has no neutron at all and is by far the most abundant form. Deuterium, hydrogen-2, written D, has one neutron, so its atom is about twice as massive as an ordinary hydrogen atom, and that doubling of mass is what earned it the name heavy hydrogen. Tritium, hydrogen-3, has two neutrons and is radioactive, decaying with a half-life of about twelve years, and it occurs only in traces. The name matters in practice because of the compound it forms: water made with deuterium in place of ordinary hydrogen is heavy water, whose formula is written D2O, and heavy water is the moderator and coolant of the pressurised heavy water reactors that form the first stage of India's nuclear power programme. A moderator slows fast neutrons so that they can sustain fission in natural uranium, and heavy water performs that task while absorbing very few neutrons, which is why a reactor using it can run on unenriched uranium. Deuterium was identified by Harold Urey in the early 1930s, work for which he received the Nobel Prize in Chemistry, and it makes up only a very small fraction of natural hydrogen, of the order of a hundredth of one per cent, so heavy water has to be concentrated from ordinary water by industrial plants rather than mined.

• (1)Only (c) — Tritium is the heaviest of the three isotopes, with two neutrons, but it is not the one called heavy hydrogen. It is distinguished instead by being radioactive — the only unstable isotope of hydrogen — with a half-life of about twelve years, and it occurs in nature only in traces produced by cosmic rays in the upper atmosphere. A candidate reasoning that the heaviest isotope must be the one named heavy hydrogen will pick this option; the name, however, belongs by long-established usage to deuterium, the isotope whose compound heavy water is a staple of nuclear technology.
• (3)Only (d) — Choosing 'none of the above' asserts that no printed isotope answers the question, which cannot stand once deuterium has been recognised. An escape option is only worth taking when each substantive choice has been positively ruled out, and here the second of the three is not merely defensible but is the standard name in chemistry and in the nuclear industry alike. This option catches the candidate who cannot recall which isotope carries the name and prefers to avoid the choice rather than reason from the mass numbers, which settle it directly.
• (4)Only (a) and (c) — This option names protium and tritium and misses deuterium entirely, so it is wrong at both ends. Protium is the ordinary, lightest isotope with no neutron and could not be called heavy anything, while tritium is the radioactive isotope, notable for its instability rather than for a special name. The option also asks for two isotopes when the question, by its phrasing, expects a single one — a question that asks which isotope is called heavy hydrogen is not looking for a pair.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they occupy the same place in the periodic table and behave nearly identically in chemistry while differing in mass and in nuclear stability. Hydrogen is the textbook case because its three isotopes differ so much in relative mass — one, two and three units — that they are given individual names, which happens with no other element. Protium, with a lone proton, accounts for almost all natural hydrogen. Deuterium adds a neutron and is stable; its oxide, heavy water, is denser than ordinary water, freezes and boils at slightly higher temperatures, and is separated industrially from ordinary water. Tritium adds a second neutron, is radioactive with a half-life of about twelve years, and is produced both naturally by cosmic radiation and artificially in reactors. The nuclear significance of the isotopes runs in two directions. In fission reactors, heavy water serves as a moderator that slows neutrons without absorbing many of them, which allows natural uranium to be used as fuel — the basis of the Indian pressurised heavy water reactor fleet. In fusion, the reaction most studied for future power generation is that between deuterium and tritium, which is why both isotopes appear in discussions of thermonuclear energy.

MPSC asks about hydrogen isotopes in two registers, and both should be prepared. The first is straightforward chemistry — how many neutrons in each, which is radioactive, which is called heavy hydrogen — and it is answered from the mass numbers alone. The second is nuclear policy, where heavy water appears as the moderator of the pressurised heavy water reactor, the workhorse of India's three-stage nuclear programme, and where deuterium and tritium appear as the fuels of fusion research. Questions of the first kind can be reasoned out at the desk if the mass numbers are known, so no separate memorisation is needed beyond the three names. The statement-list packaging used here is the paper's standard wrapper and adds nothing to the difficulty, but note that it does include a 'none of the above' statement, one of only a small number of such escapes offered anywhere in this paper.

• Hydrogen has three isotopes: protium with no neutron, deuterium with one and tritium with two, all having a single proton.
• Deuterium is called heavy hydrogen because its atom is roughly twice the mass of an ordinary hydrogen atom; it is written D and is stable.
• Heavy water, D2O, is the moderator and coolant of pressurised heavy water reactors, which can therefore run on natural, unenriched uranium.
• Tritium is the only radioactive isotope of hydrogen, with a half-life of about twelve years, and occurs naturally only in traces.
• Deuterium was identified by Harold Urey in the early 1930s, work recognised with the Nobel Prize in Chemistry.

• Assuming the heaviest isotope must be the one called heavy hydrogen; the name belongs to deuterium, not to tritium
• Forgetting that tritium, not deuterium, is the radioactive isotope of hydrogen
• Taking an escape option such as 'none of the above' without first ruling out each named choice
• Confusing heavy water as a moderator with the fuel itself; the moderator slows neutrons, it does not undergo fission

Hydrogen isotopes appear in MPSC papers as one-line recall — which isotope is radioactive, which is heavy hydrogen, how many neutrons does each contain — and as a bridge into nuclear questions on reactors and moderators. Because the three names differ only by their neutron count, a candidate who has fixed the mass numbers can answer any variant without further memory. Companion items ask which reactor type India uses, what a moderator does and which two isotopes are proposed as fusion fuel, so the chemistry and the energy-policy sides of the topic are worth revising together.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which isotope of hydrogen is radioactive ?

  • (a)Protium
  • (b)Deuterium
  • (c)Tritium
  • (d)All three are radioactive

  Answer(c) Tritium — with one proton and two neutrons, it is the only unstable isotope of hydrogen and decays with a half-life of about twelve years. Protium and deuterium are both stable, deuterium being the isotope known as heavy hydrogen.

• practice — not a real PYQ

  Heavy water is used in a pressurised heavy water reactor mainly to :

  • (a)Serve as the nuclear fuel
  • (b)Slow down fast neutrons so that fission can be sustained
  • (c)Absorb neutrons and stop the chain reaction
  • (d)Shield the reactor against gamma radiation

  Answer(b) Slow down fast neutrons so that fission can be sustained — heavy water is an excellent moderator because it slows neutrons while absorbing very few of them, which allows natural uranium to be used as fuel. Absorbing neutrons to stop the reaction is the work of the control rods, not of the moderator.