I would just round to the nearest eV and keep the commas. > the mass of an electron is approximately 510,999 eV and the mass of a proton is 938,272,088 eV. Th

I would just round to the nearest eV and keep the commas. > the mass of an electron is approximately 510,999 eV and the mass of a proton is 938,272,088 eV. This reads best to me.