Why did j. j. thomson reason that electrons must be a part of the atoms of all elements?

J.Jj. Thomson proposed that electrons must be a part of the atoms of all elements because the charge-to-mass ratio of electrons were the same, regardless of the gas used. J.J. thomson is a physicist and a Nobel Prize winner. His research on cathode rays led to the discovery of the electrons and with this discovery he pursued to more researches pertaining to atomic structure particularly on the subject of subatomic particles. He was able to determine that all matter consists of particles that are smaller than what we know as atoms. He initially termed these particles as corpuscles which is now known as electrons. 

Alpha particles have positive charge, beta particles have negative charge, and answer 4 just doesn’t make sense. thus, you are left with only one possible answer.

The answer is 323 g.