<Tr> <Th> MeV / c </Th> <Td> 7002931494095400000 ♠ 931.494 0954 (57) </Td> </Tr> <Tr> <Th> m </Th> <Td> 7003182288848619200 ♠ 1822. 888 486 192 (53) </Td> </Tr> <P> The unified atomic mass unit or dalton (symbol: u, or Da) is a standard unit of mass that quantifies 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 at rest, and has a value of 6973166053904000000 ♠ 1.660 539 040 (20) × 10 kg, or approximately 1.66 yoctograms . 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 . </P> <P> 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 . </P>

What does the u symbol mean in chemistry
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