unified atomic mass unit

physics. Symbols u, m_u. ¹/₁₂ the mass of an atom of the commonest carbon isotope (¹²₆C) = 1.660 538 86(28) × 10^-27 kg with relative standard uncertainty 1.7 × 10^-7.
[Mohr P. J., Taylor B. N. CODATA Recommended Values of the Fundamental Physical Constants: 2002 (to be published)]
[Mohr P. J., Taylor B. N. Rev. Mod. Phys. Vol. 72:351-495 (2000)]
[Mohr P. Phys. Today Vol. 53:7, 11-16 (2000)]
[For latest recommended values, see http://physics.nist.gov/cuu/Constants/index.html] Its frequent use in the burgeoning world of molecular biology has prompted the use of the handier name dalton.

History

Imprecisely the mass of a proton else neutron, the unit began as the atomic mass unit or amu, defined as the mass of the hydrogen atom, then, in 1885, as ¹/₁₆ the mass of an atom of oxygen. However, chemists came to see this as the naturally occurring mixture of oxygen isotopes, and physicists the pure commonest isotope, the latter giving values 0.027~% below the former. The unifying carbon-based definition and the current name were adopted in 1961 (with symbol u assigned in the SI). See mole for elaboration.