unified atomic mass unit (dalton)
The unified atomic mass unit, also called the dalton, is defined as exactly one twelfth of the mass of a free carbon-12 atom at rest in its ground state. Its value in kilograms, 1.66053906892 × 10⁻²⁷ kg, is a measured quantity, not exact, with a relative standard uncertainty of a few parts in 10¹⁰ in the 2022 CODATA adjustment.
Watch out: This is one of the few units in this group whose SI value is not exact, and the 2019 SI redefinition is the reason. Before 2019 the molar mass of carbon-12 was exactly 12 g/mol by definition, which made the dalton exactly 1 g/mol divided by the Avogadro number. Now that the Avogadro constant is fixed and the kilogram is defined through the Planck constant, the molar mass constant is measured instead, and the dalton's value in kilograms carries an experimental uncertainty. The shift is far too small to matter in chemistry but it is real.
| 1 u | 1.6605391e-27 kg |
Chemists and physicists used incompatible scales for decades, one based on natural oxygen and one on oxygen-16. The carbon-12 scale was adopted by IUPAP in 1960 and IUPAC in 1961 as a compromise both communities could accept, and it is the scale still in use.