Relative mass is a concept used in chemistry to simplify the process of working out the mass of an atom or molecule. It is defined as the mass of an atom or molecule relative to that of 1/12 of a carbon-12 atom. Scientists use the mass of a carbon-12 atom as the basis to measure the masses of all other atoms, and define the relative atomic mass of the carbon-12 atom as 12. The relative atomic mass (RAM or Ar) is the weighted average of the masses of an elements isotopes compared to the mass of a carbon-12 atom. For a single given sample, the relative atomic mass of a given element is the weighted arithmetic mean of the masses of the individual atoms (including all its isotopes) that are present in the sample. The relative atomic mass can vary significantly between samples because the samples origin may have produced combinations of isotopic abundances in varying ratios.