dalton
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A dalton is a unit of measurement used to express the mass of very tiny objects like atoms and molecules, defined as one-twelfth of the mass of a carbon-12 atom. It matters because scientists need a standard way to compare and describe the weights of individual atoms and molecules, which is essential for chemistry, biology, and physics research.
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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. It is a non-SI unit accepted for use with SI. 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(C), the atomic mass of carbon-12: mu = ma(C)/12 = 1 Da.
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.