Demcoritus was a citizen of Greece in the 4th and 5th century B.C. Although not well known in his day, he was known to Plato. His idea of an atomic model was never tested, nor it could it have been with the technology of his day.
In the early 19th century, Englishman John Dalton revived the atomic model with a five part theory that would set the course in atomic discoveries for the next 130 years:
1. Elements are made of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in their size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in integer ratios to form chemical compounds.
5. During a chemical reaction, atoms are combined, separated, or rearranged.
Cathode Ray Tube
J.J. Thomson is credited with the discovery of the electron through his experiments with a cathode ray tube. The cathode ray tube (sometimes called Crookes tube, as it was developed by William Crookes) which is illustrated in the video below, was used to make this discovery.
Gold Foil Experiment
Animations that describe how Rutherford performed the gold foil experiment:
(1) Asimov, Isaac. Atom: Journey Across the Subatomic Cosmos; Truman Talley Books: New York, 1992.
(2) Asimov, Isaac. Understanding Physics, Volume III; Dorset Press: New York, 1988.
(3) Cohen, Bernard L. The Heart of the Atom; Anchor Books: Garden City, NY, 1967.
(4) Grimes, Robin W. and Nuttall, William J. Science. 2010, 329, 799-803.
(5) Kaplan, Irving. Nuclear Physics, 2nd ed.; Addison-Wesley: Reading, MA, 1964.
(6) Romer, Alfred. The Restless Atom; Anchor Books: Garden City, NY, 1960.
(7) Smith, Timothy Paul. The Anatomy of a Neutron. American Scientist. 2010, 98, 478-485.