Isotopes and Atomic Mass
~8 min read
- Isotopes: Same Z, different A. Same chemistry, different mass.
- Common examples: Hydrogen: ¹H, ²H (deuterium), ³H (tritium). Carbon: C-12 (98.9%), C-13, C-14. Uranium: U-235, U-238.
- Uses: C-14 (radiocarbon dating), U-235 (fission reactor), I-131 (thyroid treatment), Co-60 (cancer treatment).
Isotopes are atoms of the same element with different mass numbers. NDA tests common isotopes and their applications.
Isotopes — Definition
- Atoms of the same element with same atomic number (Z) but different mass numbers (A).
- Same protons, same chemistry, different neutrons.
- Example: ₁¹H (no neutrons), ₁²H (1 neutron, called deuterium), ₁³H (2 neutrons, tritium).
- Isobars: Different Z, same A. ⁴⁰Ar and ⁴⁰K.
- Isotones: Same number of neutrons. ¹⁴C and ¹⁶O (both have 8 neutrons).
Common Isotopes
| Element | Isotopes | Notes |
|---|---|---|
| Hydrogen | ¹H (protium, 99.98%), ²H (deuterium, 0.02%), ³H (tritium, trace) | Deuterium → heavy water (D₂O) |
| Carbon | ¹²C (98.9%), ¹³C (1.1%), ¹⁴C (trace) | C-14 used in radiocarbon dating |
| Oxygen | ¹⁶O (99.76%), ¹⁷O, ¹⁸O | O-18 used in climate research |
| Uranium | U-235 (0.7%), U-238 (99.3%) | U-235 is fissile (reactor fuel) |
| Iodine | I-127 (stable), I-131 (radioactive) | I-131 used for thyroid scans/therapy |
| Cobalt | Co-59 (stable), Co-60 (radioactive) | Co-60 used in cancer radiotherapy |
Atomic Mass and Average
- Most elements occur as a mix of isotopes. The "atomic mass" on periodic table is the weighted average.
- Chlorine atomic mass = 35.5 because it's ~75% Cl-35 and ~25% Cl-37.
- Atomic mass unit (amu or u): 1/12 mass of one C-12 atom = 1.66 × 10⁻²⁷ kg.
- Avogadro's number: 6.022 × 10²³ — number of atoms in 1 mole.
- Mole: 1 mole = mass equal to atomic/molecular mass in grams. (1 mole H = 1 g; 1 mole CO₂ = 44 g.)
Applications of Isotopes
- C-14 (Radiocarbon dating): Living organisms have constant C-14/C-12 ratio. After death, C-14 decays (half-life 5,730 yrs). Measuring residual ratio dates the specimen.
- U-235: Nuclear reactor fuel (sustained fission).
- I-131: Thyroid imaging and treatment (concentrates in thyroid).
- Co-60: Cancer radiotherapy. Gamma rays kill cancer cells.
- P-32: Tagged in biological research to trace phosphorus metabolism.
- Tc-99m: Medical imaging (most-used radioisotope in nuclear medicine).
- Deuterium (D₂O — heavy water): Moderator in nuclear reactors (CIRUS, Dhruva at BARC).
NDA PYQ Examples
Q: Heavy water is:
(a) Water at high pressure (b) Polluted water (c) Water with deuterium (D₂O) (d) Boiling water
Answer: (c) Water with deuterium (D₂O).
Q: Carbon-14 is used in:
(a) Nuclear reactor (b) Cancer treatment (c) Radiocarbon dating (d) Thyroid imaging
Answer: (c) Radiocarbon dating.
Q: Atomic mass of chlorine is 35.5 because:
(a) Half its electrons are in each shell (b) Mix of Cl-35 and Cl-37 isotopes (c) Loses half its mass in reaction (d) Avg of proton and neutron
Answer: (b) Mix of Cl-35 and Cl-37 isotopes (~75:25).
Q: Avogadro's number is approximately:
(a) 6.02 × 10²² (b) 6.02 × 10²³ (c) 6.02 × 10²⁴ (d) 6.02 × 10²⁵
Answer: (b) 6.02 × 10²³ per mole.
Drill Isotopes and Atomic Mass for NDA
NDA-pattern items on Isotopes and Atomic Mass with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
What's the difference between isotope and isobar?
Isotopes: same element (same Z), different mass. Isobars: different elements (different Z), same mass. ¹⁴N and ¹⁴C: both have A=14, but N has Z=7 and C has Z=6, so they're isobars.
Why is heavy water used in nuclear reactors?
D₂O (heavy water) is an excellent neutron moderator — slows fast neutrons to thermal energies needed for sustained U-235 fission. India's PHWR (Pressurised Heavy Water Reactor) design uses it.
Why is C-14 useful for dating?
Constant atmospheric C-14/C-12 ratio gets incorporated into living organisms. Death stops the exchange. C-14 decays with half-life 5,730 years. Measuring remaining C-14 dates the remains. Effective up to ~50,000 years.
How is uranium enriched for reactors?
Natural uranium is 99.3% U-238 and 0.7% U-235. Reactors need 3-5% U-235; weapons need ~90%. Enrichment uses gas centrifuges (most common today) or gaseous diffusion to separate by mass.
What is a mole?
Avogadro's number (6.022 × 10²³) of particles. Equal to atomic/molecular mass expressed in grams. 1 mole of any substance contains the same number of particles. Lets us scale from atoms to laboratory quantities.