With reference to 'Isotopes', which of the following statements is/are correct ? 1. Atoms with identical atomic numbers but different atomic mass numbers are known as isotopes. 2. Protium, Deuterium and Tritium are isotopes of carbon. Select the correct answer from the code given below : Code :
- (a)Only 2
- (b)Both 1 and 2
- (c)Neither 1 nor 2
- (d)Only 1
Correct — D, Only 1. Statement 1 is the textbook definition and it is exactly right: isotopes are atoms of the same element that have the same atomic number (the same number of protons, which is what fixes the element) but different mass numbers, because they carry different numbers of neutrons. Statement 2 is false: protium, deuterium and tritium are the three isotopes of HYDROGEN, not of carbon. All three have atomic number 1 — protium has no neutron and mass number 1, deuterium has one neutron and mass number 2, tritium has two neutrons and mass number 3. Carbon's isotopes are carbon-12, carbon-13 and carbon-14, all with atomic number 6. So statement 1 alone is correct.
- (a)Only 2 — This rejects the correct definition and accepts the false example — the exact opposite of the truth. Statement 1's wording, 'identical atomic numbers but different atomic mass numbers', is the standard definition of isotopes and cannot be faulted.
- (b)Both 1 and 2 — The most tempting wrong answer, because statement 1 is plainly correct and a hurried reader carries that confidence into statement 2. But protium, deuterium and tritium are hydrogen's isotopes; the trio has nothing to do with carbon, whose isotopes are C-12, C-13 and C-14.
- (c)Neither 1 nor 2 — Statement 1 is true, so 'neither' cannot hold. Only statement 2, which misassigns hydrogen's isotopes to carbon, is wrong.
An atom's identity is set by its atomic number Z, the number of protons in the nucleus; the mass number A is protons plus neutrons. Vary the neutron count while holding the proton count fixed and you get isotopes — same element, same chemistry, different mass and different nuclear stability. Hold A fixed and vary Z instead and you get isobars (argon-40 and calcium-40, for instance). Because isotopes have the same electron arrangement, they are chemically almost indistinguishable; they differ in physical properties such as density, rate of diffusion and radioactivity. The word 'isotope' was coined in 1913 by Frederick Soddy.
The question is built on a single swap. It states the definition correctly and then attaches hydrogen's isotope names to carbon, betting that candidates who remember 'carbon-14 dating' will have carbon at the front of their minds when they see the word 'isotope'. The defence is to remember what the names literally mean: protium is 'first' (a lone proton), deuterium is 'second' (proton plus one neutron) and tritium is 'third' (proton plus two neutrons) — a counting sequence that only makes sense for hydrogen, the element with a single proton. Hydrogen is the only element whose isotopes have distinct names at all.
- Hydrogen has three isotopes: protium (1 proton, 0 neutrons, mass number 1, over 99.9 per cent of natural hydrogen), deuterium (1 neutron, mass number 2) and tritium (2 neutrons, mass number 3); only tritium is radioactive, with a half-life of about 12.3 years
- Carbon's isotopes are carbon-12 and carbon-13, both stable, and carbon-14, which is radioactive with a half-life of about 5,730 years and is the basis of radiocarbon dating
- Deuterium oxide (D2O) is heavy water, the moderator in India's pressurised heavy-water reactors; deuterium and tritium are the fuel of nuclear fusion in thermonuclear weapons and in fusion research
- Isotopes share the same number of electrons and so behave almost identically in chemical reactions; isobars, by contrast, have the same mass number but different atomic numbers, and isotones have the same number of neutrons
- Most naturally occurring isotopes are stable — being an isotope does not by itself mean being radioactive
Statement 1's definition is exactly this table. But the named trio belongs to hydrogen, not carbon, so statement 2 fails — leaving option (d), Only 1.
- Attaching protium, deuterium and tritium to carbon because carbon-14 dating is the most familiar isotope story
- Confusing isotopes (same atomic number, different mass number) with isobars (same mass number, different atomic number)
- Assuming every isotope is radioactive — carbon-12, carbon-13, protium and deuterium are all stable
UPPSC asks the definition and the standard examples head-on, often as a two-statement item like this one; UPSC prefers the application layer — which isotope is used for what, how a scintillation counter detects a gamma emitter, or what heavy water does inside a reactor.
Which one of the following is NOT radioactive?
- (a) Astatine
- (b) Francium
- (c) Tritium
- (d) Zirconium
Answer(d) Zirconium
Same concept, tested from the nuclear-stability side. Its explanation turns on tritium being a beta-emitting isotope of hydrogen and on zirconium having several stable isotopes — the very distinction between hydrogen's isotopes and stable isotopes generally that statement 2 here abuses.
The function of heavy water in a nuclear reactor is to
- (a) Slow down the speed of neutrons
- (b) Increase the speed of neutrons
- (c) Cool down the reactor
- (d) Stop the nuclear reaction
Answer(a) Slow down the speed of neutrons
Heavy water is deuterium oxide, so this question is about what one of hydrogen's isotopes is actually used for. It is the standard follow-up UPSC asks once a candidate knows what deuterium is.
With reference to the radioactivity, which of the following statements is/are correct ? 1. Radioactivity is a nuclear property. 2. Hydrogen bomb is prepared on the principle of nuclear fission. Select the correct answer using the codes given below. Codes :
- (a) Only 1
- (b) Only 2
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(a) Only 1
The same hydrogen isotopes on the other side of the physics: a hydrogen bomb works by fusing deuterium and tritium, which is why the statement calling it a fission device is false. UPPSC has been mining this small cluster of hydrogen-isotope facts across cycles.
- practice — not a real PYQ
Atoms having the same mass number but different atomic numbers are called
- (a)Isotopes
- (b)Isobars
- (c)Isotones
- (d)Isomers
Answer(b) Isobars — for example argon-40 and calcium-40, which share mass number 40 but differ in proton count.
- practice — not a real PYQ
Heavy water, used as a moderator in nuclear reactors, is an oxide of which isotope of hydrogen?
- (a)Protium
- (b)Deuterium
- (c)Tritium
- (d)Helium-3
Answer(b) Deuterium — heavy water is deuterium oxide, D2O.