the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.99264648×10⁻²³gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to 1.66053873×10⁻²⁴gram.

The observed atomic mass is slightly less than the sum of the masses of the protons (proton), neutrons (neutron), and electrons (electron) that make up the atom. The difference, called the mass defect, is accounted for during the combination of these particles by conversion into binding energy, according to an equation in which the energy (E) released equals the product of the mass (m) consumed and the square of the velocity of light in vacuum (c); thus, E=mc². See also atomic weight.