Atomic weight

Atomic weight, also atomic mass, is the weight of an atom of any element expressed in atomic mass units. Formerly it was compared to that of hydrogen, the unit, however, in modern times is compared to an atom of carbon-12. Atomic weight is approximately equivalent to the number of protons and neutrons in the atom. Hydrogen has the atomic weight 1 (1 proton + 0 neutron), that is why it was formerly considered a atomic mass unit.