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5 BCE
Democritus concept of the Atom
Democritus proposed the existence of atoms. Limited technology at the time meant this was just an idea and could not be tested. His mentor, Leucippus, originally came up with the atomic theory, but it was then adopted by Democritus. The atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” According to Democritus atoms were miniscule quantities of matter. -
Dalton’s Atomic Theory
Dalton developed his theory of atoms and matter. Experiments in chemistry and careful measurement of results allowed Dalton to support his hypotheses with evidence. Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton (1766-1844). Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought. -
Thomson and the “Plum Pudding” model of the Atom
JJ Thomson discovered the electron. Experiment using electric charges in cathode ray tubes. In Thomson’s model, the atom is composed of electrons (which Thomson still called “corpuscles,” though G. J. Stoney had proposed that atoms of electricity be called electrons in 1894) surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”. -
Rutherford's planetary model
Rutherford developed the planetary model of the atom. Experiment using alpha particles fired at thin gold foil and an early prototype Geiger counter to measure results. Rutherford's new model for the atom, based on the experimental results, contained the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom. This region would be known as the "nucleus" of the atom. -
Bohr's model
Bohr developed a model of the atom. Bohr used mathematical models and laws of quantum physics to propose his updated model. The Bohr model of the hydrogen atom or a hydrogen-like ion, where the negatively- charged electron confined to an atomic shell encircles a small, positively charged atomic nucleus and where an electron jumps between orbits it is accompanied by an emitted or absorbed amount of electromagnetic energy. -
Chadwick-Bohr model
The neutron was discovered by James Chadwick. Experiments using kinematics and accurate measurements lead to proving the existence of a previously undetected subatomic particle, the neutron. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.