In chemical scale, the relative mass of the isotopic mixture of oxygen atoms $\left({\mathrm{O}}^{16},{\mathrm{O}}^{17},{\mathrm{O}}^{18}\right)$ is assumed to be equal to

16.002 is the relative mass at which isotopic mixture of oxygen atoms is assumed to be equal. The relative mass is compared on the bases of specific mass of the atom.