dalton
Non-SI unit accepted for use with SI
| Name | Symbol | Quantity | Value in SI units | |
| dalton | Da | mass | 1.660 539 040 (20) × 10−27 kg 1.660 539 040 (20) yg |
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DefinitionThe dalton, symbol Da, or unified atomic mass unit, is a non-SI unit of mass accepted for use with the SI. One dalton is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. In SI units, one dalton is 1.660 539 040 (20) × 10−27 kg, or approximately 1.66 yoctograms. |
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The dalton is used as a unit of mass for quantities on an atomic or molecular scale. One dalton is approximately the mass of one nucleon (either a single proton or neutron), and is approximately equivalent to 1 g mol-1.
Examples of atomic and molecular masses
| Name | Symbol/formula | Mass in Da | Mass in SI units |
| hydrogen | H | 1.00 | 1.66 yg |
| carbon-12 | 12C | 12.00 | 19.93 yg |
| silicon-28 | 28Si | 27.98 | 46.46 yg |
| iron-56 | 56Fe | 55.93 | 92.88 yg |
| uranium-235 | 235U | 235.04 | 390.29 yg |
| water | H2O | 18.01 | 29.91 yg |
| carbon dioxide | CO2 | 44.01 | 73.08 yg |
| ethanol | C2H5OH | 46.07 | 76.50 yg |
| benzene | C6H6 | 78.11 | 129.70 yg |
| human haemoglobin | ≈ 64 000 | ≈ 106 000 yg |
History
Prior to 2019, the mole was defined in terms of the kilogram and the dalton, and was equal to
“the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12”.
The numerical value of the mole is now defined exactly, and the value of the dalton in SI units must be obtained experimentally.
Metric System 