Relative atomic mass

The mean mass of an element’s atoms compared with one-twelfth of the mass of a carbon-12 atom, taking account of the abundance of each isotope. Its symbol is $A_{\mathrm r}$ and it has no unit.

The mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom. Its symbol is $A_r$, and it has no unit.

The weighted mean of the relative masses of an element’s atoms, taking account of the abundances of its isotopes. Its symbol is $A_\mathrm{r}$ and it has no unit.

The weighted mean mass of an element’s atoms compared with one-twelfth of the mass of a carbon-12 atom; it depends on the masses and relative abundances of its isotopes.

The average mass of an atom of an element on a relative scale, taking account of its isotopes and their abundances. Its symbol is $A_r$.