Isotopes, Isobars and Isotones
~8 min read
- Isotopes: Same Z, different A. Same chemistry, different mass.
- Isobars: Different Z, same A. Different elements, same mass number.
- Isotones: Same number of neutrons. Different Z and A.
Atoms with the same proton count but different neutrons are isotopes; CDS/OTA contrasts them with isobars and isotones, and tests applications like carbon-14 dating and uranium fission.
Definitions
| Term | Same | Different | Example |
|---|---|---|---|
| Isotopes | Atomic number Z | Mass number A | ¹H, ²H, ³H |
| Isobars | Mass number A | Atomic number Z | ⁴⁰Ar and ⁴⁰K |
| Isotones | Number of neutrons | Z and A | ¹⁴C (6p, 8n) and ¹⁶O (8p, 8n) |
Common Isotopes and Uses
| Isotope | Use |
|---|---|
| Hydrogen — ¹H, ²H (Deuterium), ³H (Tritium) | Heavy water D₂O — moderator in nuclear reactors. Tritium — H-bomb, luminous dials. |
| Carbon-14 | Radiocarbon dating (half-life 5,730 yr). |
| Uranium-235 | Fissile fuel in nuclear reactors and bombs. |
| Uranium-238 | 99.3% of natural uranium; fertile (breeds plutonium-239). |
| Iodine-131 | Thyroid imaging and treatment of hyperthyroidism. |
| Cobalt-60 | Cancer radiotherapy; food sterilisation. |
| Phosphorus-32 | Tracer in agricultural and biological research. |
| Technetium-99m | Most-used radioisotope in medical imaging. |
Average Atomic Mass and Mole
- Atomic mass on the periodic table is a weighted average of isotopes.
- Chlorine atomic mass is 35.5 because it is ~75% Cl-35 and ~25% Cl-37.
- 1 u = 1/12 mass of C-12 atom = 1.66 × 10⁻²⁷ kg.
- Avogadro's number = 6.022 × 10²³ — number of atoms in 1 mole.
- 1 mole of any substance has mass equal to its atomic / molecular mass in grams.
CDS/OTA PYQ Examples
Q: Heavy water is:
(a) Water at high pressure (b) Water with deuterium (D₂O) (c) Polluted water (d) Boiling water
Answer: (b) D₂O — used as a neutron moderator.
Q: Carbon-14 is used in:
(a) Nuclear power plants (b) Cancer treatment (c) Radiocarbon dating (d) Thyroid imaging
Answer: (c) Radiocarbon dating.
Q: Atomic mass of chlorine (35.5) is fractional because:
(a) Half its electrons are shared (b) It loses half its mass in reaction (c) It is a mixture of Cl-35 and Cl-37 isotopes (d) Mass is averaged with hydrogen
Answer: (c) Mixture of isotopes (~3:1).
Q: Avogadro's number is:
(a) 6.022 × 10²² (b) 6.022 × 10²³ (c) 6.022 × 10²⁴ (d) 6.022 × 10²⁵
Answer: (b) 6.022 × 10²³ /mol.
Drill Isotopes, Isobars and Isotones for CDS/OTA
CDS/OTA-pattern items on Isotopes, Isobars and Isotones with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Do isotopes have the same chemistry?
Yes — chemistry depends on electrons (equal to protons in a neutral atom). Isotopes differ only in neutrons, so their chemical behaviour is essentially identical.
How does radiocarbon dating work?
Living organisms maintain a constant C-14/C-12 ratio. On death, C-14 decays with half-life 5,730 yr. Measuring residual C-14 dates organic remains up to ~50,000 yr.
Why is heavy water used in PHWRs?
D₂O slows fast neutrons to thermal energies needed for U-235 fission without absorbing them, allowing natural uranium (not enriched) to sustain a chain reaction.