The mass of a neutral Carbon-12 atom is exactly 12 u, which means it includes the bound mass of protons and neutrons, as well as the mass of the electrons. This would contain 1.40% (\(\dfrac{1.40}{100}\) 1 mol) \({}_{\text{82}}^{\text{204}}\text{Pb}\) whose molar mass is 203.973 g mol1. &= 19.92\cdot 10^{-24}~\mathrm{g}\\ Multiply the exact mass of each isotope by its corresponding mass fraction (percent abundance 100) to obtain its weighted mass. In general, we can write, Bromine has only two isotopes. We can also assume that it has 8 electrons. /*