The isotopes of hydrogen and their respective mass ratio are _________.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (b) 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.  (c) 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.

Concept: 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.

State PCS 2025 Q70 — Science & Technology PYQ with Answer

State PCS 2025 Q70 — Science & Technology PYQ with Answer