Unified Atomic Mass Units
Key Stage 5
Meaning
Unified atomic mass units (u or Da) is an SI unit equivalent to the mass of 1/12th the mass of a Carbon atom.
About Unified Atomic Mass Units
- Unified atomic mass units may be denoted with a lower case 'u' or with an upper case 'D', followed by a lower case 'a'.
- 1 unified atomic mass unit is approximately equal to the mass of a free nucleon (proton or neutron).
- A free nucleon has a mass slightly greater than 1u, but when a nucleon is bound within a nucleus some of the mass of the free nucleon has been converted into energy due to the mass energy equivalence, giving it a slightly smaller mass.
Examples
| Particle | Rest Mass / kg | Unified Atomic Mass Units | Rest Energy / MeV |
| Proton (\(p\)) | \(1.67\times10^{-27}\) | \(1.007\) | \(938\) |
| Neutron (\(n\)) | \(1.67\times10^{-27}\) | \(1.009\) | \(939\) |
| Electron (\(e\)) | \(9.11\times10^{-31}\) | \(0.05486\) | \(5.11\) |
| Muon (\(\mu\)) | \(1.88\times10^{-28}\) | \(0.1134\) | \(106\) |
| π-meson (\(\pi^±\)) | \(2.49\times10^{-28}\) | \(0.1500\) | \(140\) |
| π-meson (\(\pi^0\)) | \(2.41\times10^{-28}\) | \(0.1449\) | \(135\) |
| K-meson (\(K^±\)) | \(8.80\times10^{-28}\) | \(0.5300\) | \(494\) |
| K-meson (\(K^0\)) | \(8.87\times10^{-28}\) | \(0.5442\) | \(498\) |
| Photon (\(\gamma\)) | \(0\) | \(0\) | \(0\) |
| Neutrino (\(\nu\)) | \(0\) | \(0\) | \(0\) |