3.8.1.6
The atomic mass unit (u) is the standard unit for nuclear masses
Nuclear Energy & Binding Energy — AQA A-Level Physics
Key Definition
Unified atomic mass unit (u) — Defined as exactly one-twelfth of the mass of a carbon-12 atom. 1 u = 1.661 × 10⁻²⁷ kg.
- The mass of any atom in u is approximately equal to its nucleon number A.
- Proton mass ≈ 1 u, neutron mass ≈ 1 u, electron mass ≈ 0.0005 u, alpha particle mass ≈ 4 u.
- 1 u is equivalent to 931.5 MeV (via $E = mc^{2}). This conversion$ is on the data sheet.
- Masses can be quoted in kg, u, or MeV/c² — all are interchangeable.
Worked Example
Estimate the mass of a copernicium-285 nucleus in kg.
Show Solution
1
Mass in u equals the nucleon number
Mass ≈ 285 u
2
Convert to kg
$$m = 285 \times 1.66 \times 10^{-27} = 4.73 \times 10^{-25} \text{ kg}$$
Answer
$m = 4.73 \times 10^{-25}$ kg