1897, Discovery of Electrons

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Year: 1897
Chemist: Joseph John Thomson
Contribution: Discovery of Electrons

By trade J.J. Thomson was actually a physicist, but that didn't keep him from discovering the electron in the year 1897 while studying electric discharges. He interpreted electrons as significantly smaller than the nucleus, and calculated a large charge-to-mass ratio.

Electrons, of course, are the  instigators of any chemical reaction by their exchange or allotment. Because of their vitality to the atom itself, their discovery was an important contribution as later chemists were better able to establish the structure of the atom.

Information Credit: Chemical Heritage Foundation