The atomic mass (m a or m) is the mass of an atom.Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1 Da is defined as 1 ⁄ 12 of the mass of a free carbon-12 atom at rest in its ground state.
The word relative in relative atomic mass refers to this scaling relative to carbon-12. Relative atomic mass values are ratios;:1 relative atomic mass is a dimensionless quantity. Relative atomic mass is the same as atomic weight, which is the older term. The number of protons an atom has determines what element it is.