Dalton (unit)

The dalton (symbol: Da), or unified atomic mass unit (symbol: u), is a unit of mass defined as ⁠1/12⁠ of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The word "unified" emphasizes that the definition was jointly accepted by physics (IUPAP) and chemistry (IUPAC) organizations. The atomic mass constant, denoted mu, is an atomic-scale reference mass, defined identically, but it is not a unit of mass. Expressed in terms of ma(12C), the atomic mass of carbon-12: mu = ma(12C)/12 = 1 Da. A proton has a mass that is approximately equal to 1 dalton. The value serves as a conversion factor of mass from daltons to kilograms, which can easily be converted to grams and other metric units of mass. The 2019 revision of the SI redefined the kilogram by fixing the value of the Planck constant (h), improving the precision of the atomic mass constant expressed in SI units by anchoring it to fixed physical constants. Although the dalton remains defined via carbon-12, the revision enhances traceability and accuracy in atomic mass measurements. The dalton's numerical value in terms of the fixed-h kilogram is an experimentally determined quantity that, along with its inherent uncertainty, is updated periodically. The 2022 CODATA recommended value of the atomic mass constant expressed in the SI base unit kilogram is:mu = 1.66053906892(52)×10−27 kg. The previous 2018 CODATA value was used in the traditional definition of the Avogadro number to obtain the value g/Da = 6.0221407620(18)×1023, which was then rounded to 9 significant figures (thus guaranteeing continuity, to the precision indicated) and used to define it at exactly that value for the 2019 redefinition of the mole. The mole is a unit of amount of substance used in chemistry and physics, such that the...