Definition of 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). One
unified atomic mass unit is approximately
the mass of one nucleon (either a single proton or neutron) and is numerically
equivalent to 1 g/mol. 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 1.660539040(20)×10−27 kg.[3] The CIPM has 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, many sources still use the term "amu" but now define it in
the same way as u (i.e., based on carbon-12). In this sense, most uses of the
terms "atomic mass units" and "amu" today actually refer to
unified atomic mass unit. For standardization a specific atomic nucleus
(carbon-12 vs. oxygen-16) had to be chosen because the average mass of a
nucleon depends on the count of the nucleons in the atomic nucleus due to mass
defect. This is also why the mass of a proton or neutron by itself is more than
(and not equal to) 1 u.
The atomic mass unit is not the unit of mass in the
atomic units system, which is rather the electron rest mass (me).
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