The "Relative Mass Excess" is the mass excess between the monoisotopic mass of an element and the mass of its A + 1 or its A + 2 isotopic cluster (extrapolated from relative mass defect).
Applicable units are those of quantitykind:RelativeMassExcess
http://en.wikipedia.org/wiki/Mass_excess
The "Relative Mass Excess" is the mass excess between the monoisotopic mass of an element and the mass of its A + 1 or its A + 2 isotopic cluster (extrapolated from relative mass defect).
$\Delta_r = \frac{\Delta}{m_u}$, where $\Delta$ is the mass excess and $m_u$ is the unified atomic mass constant.
http://www.iso.org/iso/catalogue_detail?csnumber=31895