What has the lowest mass of any charged leptop ?
This fact was historically important in convincing Rutherford of the importance of Bohr 's model , for it explained the fact that the frequencies of lines in the spectra for singly ionized helium do not differ from those of hydrogen by a factor of exactly 4 , but rather by 4 times the ratio of the reduced mass for the hydrogen vs. the helium systems , which was much closer to the experimental ratio than exactly 4.0.