<P> The atomic mass (m) is the mass of an atom . Its unit is the unified atomic mass units (symbol: u, or Da) where 1 unified atomic mass unit is defined as ​ ⁄ of the mass of a single carbon - 12 atom, at rest . For atoms, the protons and neutrons of the nucleus account for almost all of the mass, and the atomic mass measured in u has nearly the same value as the mass number . </P> <P> When divided by unified atomic mass units or daltons to form a pure numeric ratio, the atomic mass of an atom becomes a dimensionless value called the relative isotopic mass (see section below). Thus, the atomic mass of a carbon - 12 atom is 12 u or 12 daltons (Da), but the relative isotopic mass of a carbon - 12 atom is simply 12 . </P>

Where does an atom's atomic mass come from