In 1897, he found cathode rays could be deflected by magnets or electrically charged plates. Through his experiment, Thomson showed cathode rays are collections of small negatively charged particles, which he named electrons . He further found they were nearly 2000 times lighter than a hydrogen atom, the lightest element. Because he found that all cathode rays were composed of electrons, he concluded that electrons must be a fundamental part of the atom. In the model, electrons were distributed throughout an atomic material much like the fruit in a plum pudding or chocolate chips in a cookie.
Atoms are normally electrically neutral.
Because the electrons were found as negatively charged, scientists deduced that some positive charge source must be present to off-set the negative. Experimental evidence soon led to the discovery of the proton which is a positively charged subatomic particle that is approximately 1840 times heavier than an electron. It was not until 1932 that Sir James Chadwick found and confirmed evidence of the neutron , or the electrically neutral subatomic particle. The discovery of the nucleus is one example of an ingenious indirect measurement technique that students can readily appreciate. A fluorescent screen was positioned around the gold foil set-up.