Molecular mass Molecular mass Relative molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the relative atomic masses of each constituent element multiplied by the number of atoms of that element in the molecular formula. The molecular mass of small to medium size molecules, measured by mass spectrometry, determines stoichiometry. For large molecules such as proteins, methods based on viscosity and light-scattering can be used to determine molecular mass when crystallographic data are not available. Both atomic and molecular masses are usually obtained relative to the mass of the isotope C (carbon 12),
Standard atomic weight "atomic" either, as it does not correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense. IUPAC publishes one formal value for each stable element, called the "standard atomic weight". Any updates are published biannually (in uneven years). The last change was published in 2015, setting a new value for ytterbium In 2017, no changes were published. The value published can have and uncertainty be an "interval" like for neon: , or can be an interval, like for boron: [10.806, 10.821]. Next to these 84 values, IUPAC also