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),
Isotopes of roentgenium Isotopes of roentgenium Roentgenium (Rg) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was Rg in 1994, which is also the only directly synthesized isotope; all others are decay products of nihonium, moscovium, and tennessine, and possibly copernicium, flerovium, and livermorium. There are 7 known radioisotopes from Rg to Rg. The longest-lived isotope is Rg with a half-life of 2.1 minutes, although the unconfirmed Rg and Rg may have a longer half-life of about 5.1 minutes and 10.7 minutes respectively.