atomic mass unit
calculation of atomic mass
TITLE: atom: Atomic mass and isotopes
SECTION: Atomic mass and isotopes
The mass of atoms is measured in terms of the atomic mass unit, which is defined to be 1/12 of the mass of an atom of carbon-12, or 1.660538921 × 10−24 gram. The mass of an atom consists of the mass of the nucleus plus that of the electrons, so the atomic mass unit is not exactly the same as the mass of the proton or neutron.