Long ago in 442 BC, Democritus came up with a theory that matter is made of small particles which he called atomos (indivisible). He also believed that atoms of the same material were the same and could not be destroyed
Charles-Augustin de Coulomb built upon Democritus's theory to say that between particles there must be a force between electrical charges. The electric charge unit was named after him.
Dalton also built upon Democritus's theory and ultimately went further in depth with Democritus's. What Dalton added to Democritus's theory was that compounds have definite ratios and were made of combinations of different atoms. He also came up with the law of conservation of mass, definite proportion, and multiple proportion.
Sir William Crookes invented the partially vacuumed electrical discharge tube where streams of electrons were discovered.
Marie Curie concluded that radioactive atoms are radioactive themselves not by how they are arranged into molecules. Both of the Curies won a Nobel prize in 1903 in Physics for discovering radioactivity.
Joseph John Thomson conducted experiments on the electric discharge in the cathode ray tubes created by Sir William Crookes. Thomson supported the hypothesis that the particles that make up cathode rays are negatively charged. Thomson concluded that all cathode rays are made of identical electrons which led him to the creation of the plum pudding model.