MPSC 2025 Prelims Paper-I · Question 70 of 100

Correct — option (4), Protium, Deuterium, Tritium; 1 : 2 : 3. Hydrogen has three naturally occurring isotopes, all with the same single proton and single electron (which is what makes each of them hydrogen), but differing in the number of neutrons in the nucleus, and therefore in mass number. Protium, the ordinary and overwhelmingly most abundant form of hydrogen, has zero neutrons and a mass number of 1. Deuterium has one neutron in addition to the proton, giving it a mass number of 2 — it is the isotope found in 'heavy water' (D2O). Tritium has two neutrons in addition to the proton, giving it a mass number of 3; unlike the other two, tritium is radioactive, decaying by beta emission with a half-life of about 12.3 years. Listed in this order — protium, deuterium, tritium — their mass numbers run 1, 2, 3, exactly matching the ratio 1 : 2 : 3 given in option (4). Both the naming order and the numeric ratio must be correct together for the statement to hold, which is why this is the only option that gets both right.

• (1)Deuterium, Protium, Tritium; 1 : 2 : 3 — This keeps the correct 1:2:3 ratio but assigns it to the wrong naming order. Listed as deuterium, protium, tritium, the ratio 1:2:3 would incorrectly imply deuterium has mass 1 (it actually has mass 2, from one proton plus one neutron) and protium has mass 2 (it actually has mass 1, from a lone proton with no neutron). Getting the ratio right while swapping the two lightest isotopes' names still produces a false statement, since a match-up requires both the names and the numbers to align correctly, in order.
• (2)Tritium, Deuterium, Protium; 2 : 3 : 1 — This option reverses the natural order of the isotopes (heaviest to lightest) and pairs it with a ratio that does not correspond correctly either: as listed, it implies tritium has mass 2 (tritium actually has mass 3, from one proton plus two neutrons), deuterium has mass 3 (deuterium actually has mass 2), and protium has mass 1 (this part is correct, but isolated). With two of the three name-to-number assignments wrong, the option fails.
• (3)Tritium, Protium, Deuterium; 2 : 3 : 1 — This option scrambles both the names and the ratio: it implies tritium has mass 2 (it actually has mass 3), protium has mass 3 (it actually has mass 1, the lightest of the three), and deuterium has mass 1 (it actually has mass 2). None of the three isotopes is correctly matched to its actual mass number in this arrangement.

Isotopes are atoms of the same element that share the same number of protons (and therefore the same atomic number and chemical identity) but differ in the number of neutrons, giving them different mass numbers. Hydrogen is the textbook example used to introduce this idea because its three isotopes have distinct, commonly used names rather than being referred to only by mass number, as most other elements' isotopes are. Protium (1H), with no neutrons, makes up more than 99.98% of naturally occurring hydrogen. Deuterium (2H, or D), with one neutron, occurs naturally in trace amounts and is used in heavy water (D2O) for applications including nuclear reactor moderation and as a tracer in chemistry and biology. Tritium (3H, or T), with two neutrons, is radioactive (a beta emitter with a half-life of about 12.3 years) and occurs only in trace natural amounts, produced mainly by cosmic-ray interactions in the upper atmosphere or artificially in nuclear reactors; it is used in applications such as self-luminous exit signs and as a fuel component in nuclear fusion research.

MPSC's basic chemistry/physics questions frequently use hydrogen's three isotopes to test whether a candidate has memorised not just their names but their correct mass-number order, since it is easy to remember 'protium, deuterium, tritium sound like they go 1, 2, 3' without being sure which name attaches to which number. The question format here, asking for both the naming order and the ratio together, specifically catches candidates who know the numbers 1:2:3 are correct in principle but have not fixed which isotope name corresponds to which number.

• Protium (1H) has 1 proton, 0 neutrons, mass number 1, and makes up over 99.98% of natural hydrogen.
• Deuterium (2H or D) has 1 proton, 1 neutron, mass number 2, and is the isotope present in heavy water (D2O).
• Tritium (3H or T) has 1 proton, 2 neutrons, mass number 3, and is radioactive with a half-life of about 12.3 years.
• All three isotopes share the same atomic number (1 proton) and are chemically hydrogen, differing only in neutron count and mass.
• Listed in increasing mass order — protium, deuterium, tritium — their mass-number ratio is 1 : 2 : 3.

• Knowing the ratio 1:2:3 is correct but assigning it to the wrong order of isotope names
• Confusing deuterium and tritium's mass numbers, since both names sound similarly 'heavy' without a fixed anchor to their actual neutron counts
• Assuming isotopes of hydrogen differ chemically as well as physically, when they are chemically identical and differ only in mass
• Forgetting that tritium, unlike protium and deuterium, is radioactive

MPSC science questions on isotopes commonly test hydrogen's three named isotopes together with their mass numbers or mass ratio, often scrambling either the name order or the ratio (or both) across the options to check whether a candidate has fixed the correct name-to-mass-number pairing rather than merely recalling that the numbers 1, 2 and 3 are involved somewhere.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which isotope of hydrogen is radioactive, decaying by beta emission with a half-life of about 12.3 years ?

  • (a)Protium
  • (b)Deuterium
  • (c)Tritium
  • (d)None of the hydrogen isotopes are radioactive

  Answer(c) Tritium — with two neutrons in addition to its single proton, tritium is the radioactive isotope of hydrogen, decaying by beta emission with a half-life of roughly 12.3 years.

• practice — not a real PYQ

  Heavy water, used as a neutron moderator in some nuclear reactors, is formed using which isotope of hydrogen in place of ordinary hydrogen ?

  • (a)Protium
  • (b)Deuterium
  • (c)Tritium
  • (d)Hydrogen-4

  Answer(b) Deuterium — heavy water (D2O) is water in which the hydrogen atoms are deuterium, the isotope with one neutron and one proton, mass number 2.