What has the lowest mass of any charged leptop ?
The combination of natural constants in the energy formula is called the Rydberg energy ( " R " E ) : formula_13 This expression is clarified by interpreting it in combinations which form more natural units : formula_14 is the rest mass energy of the electron ( 511 keV ) formula_15 is the fine structure constant formula_16 Since this derivation is with the assumption that the nucleus is orbited by one electron , we can generalize this result by letting the nucleus have a charge " q " = " Z e " where " Z " is the atomic number .