Atomic Mass Unit
The unified atomic mass unit (symbol: u) or dalton (symbol: Da) is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass). It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538921(73)×10−27 kg. One dalton is approximately equal to the mass of one nucleon; an equivalence of saying 1 g mol−1. The CIPM have categorised it as a non-SI unit accepted for use with the SI, and whose value in SI units must be obtained experimentally.
The amu without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, some technically sloppy sources still use the "amu" but now define it in the same way as u (based on carbon-12). In this sense, most use of "amu" units today actually refers to unified atomic mass units or u.
Read more about Atomic Mass Unit: History, Terminology, Relationship To SI, Examples
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