Discovery of Electrons-Cathode rays(J.J Thomson)

Many scientists contributed in revealing the presence of charged particles in an atom. It was known by 1900 that the atom was not a simple, indivisible particle but contained at least one sub-atomic particle – The electron identified by J.J Thomson. Even before the electron was identified , E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle. This sub-atomic particle had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton. In general, an electron is represented as ‘ e^– ‘ and a proton as ‘ p⁺ ‘. The mass of a proton is taken as one unit and its charge as plus one. The mass of an electron is considered to negligible and its charge is minus one.

It seemed highly likely that an atom was composed protons and electrons, mutually balancing their charges. It also appeared that the protons were in the interior of the atom, for whereas electrons could easily be peeled off but not protons.