Atomic mass constant (unified atomic mass unit)

Chemists and physicists spent decades using different atomic mass scales — oxygen-16 for physicists, natural oxygen for chemists — until the 1961 compromise on carbon-12, whose mass is exactly 12 u by definition. One dalton is 1.660 539 069 × 10⁻²⁷ kg, or 931.494 MeV/c² in energy terms, and its reciprocal is why a molar mass in g/mol and a molecular mass in daltons read as the same number.

The 2019 SI redefinition broke that identity at the tenth digit: with N_A now fixed by fiat, the molar mass constant M_u = N_A m_u is measured rather than exactly 1 g/mol, and m_u inherits a 3 × 10⁻¹⁰ relative uncertainty. The other trap is that atomic masses are not the sum of their parts — a ¹²C atom is lighter than six protons, six neutrons and six electrons by about 0.099 u, and that missing mass is precisely the nuclear binding energy.