Understanding the Atomic Weight Change of Hydrogen-3 After Decay
Question:
How does the atomic weight of Hydrogen-3 change as it decays?
Answer:
1. What is Hydrogen-3?
Hydrogen-3, also called tritium, is a radioactive isotope of hydrogen. It has:
- 1 proton
- 2 neutrons
- Atomic mass ≈ 3.0160492 u
2. Type of Decay: Beta Decay
Tritium undergoes beta (β⁻) decay to become Helium-3:
³₁H → ³₂He + e⁻ + ν̄e
This means a neutron in the tritium nucleus converts into a proton, emitting a beta particle (electron) and an antineutrino.
3. Does the Mass Number Change?
No. The number of nucleons stays the same: 3. But the atomic mass changes due to energy loss during decay.
4. Atomic Mass After Decay
- Tritium (³₁H): ~3.0160492 u
- Helium-3 (³₂He): ~3.0160293 u
5. Mass Difference (Δm)
Δm = 3.0160492 u − 3.0160293 u = 1.99 × 10⁻⁵ u
6. Why the Difference?
The small decrease in atomic mass is due to:
- Binding energy differences between Tritium and Helium-3
- Energy carried away by the beta particle and antineutrino
7. Energy Released
This mass defect converts into energy using Einstein’s equation:
ΔE = Δm × c² ≈ 18.6 keV (energy released during decay)
✅ Final Summary:
- The atomic weight of Hydrogen-3 is about 3.0160492 u.
- After beta decay, it becomes Helium-3 with a mass of 3.0160293 u.
- The atomic mass decreases slightly by ~1.99 × 10⁻⁵ u due to the conversion of mass into kinetic energy.