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335
Aristotle (384-322 BC)
Disagreed with his teacher's ideas. Believed all things were made of atoms and discarded Democritus theory. Also believed matter was infinitively divisible. He developed a theory that was based on the four elements: fire, water, earth, and air. -
400
Democritus - (460-370 BC)
First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle. Democritus learned the atomic theory from Leucippus. He never performed any experiments to prove or disprove his theory. -
John Dalton - (1776-1844)
Proposed the Law of Multiple Proportions. This law led to the proposal of the Atomic Theory in 1803. Came up with the formula for water. John Dalton also discovered color blindness, an affliction from which he suffered from also. -
Henri Becquerel - (born December 15, 1852, Paris, France and died August 25, 1908, Le Croisic)
French physicist who discovered radioactivity through his investigations of uranium, ect. For his discovery of radioactivity, Henri was awarded the Nobel Prize for physics in 1903. Before actually performing an experiment, Becquerel discovered that the photographic plates he was going to use were already exposed to the light. This led him to investigate nuclear radiation. -
J. J. Thomson - (18 December 1856 – 30 August 1940)
He was the first to suggest that the fundamental unit was over a thousand times smaller than an atom, suggesting the sub-atomic particles now known as electrons. Thomson discovered this through his explorations of cathode rays. He made the suggestion that Lenard rays could travel much further through air than an atomic-sized particle could. Proposed the "plum pudding" model. -
Ernest Rutherford - (August 30, 1871-October 19, 1937)
He worked on the behavior of the ions observed in gases which had been treated with X-rays, on ions in relation to the strength of the electric field, and related topics. -
Marie (1867-1934) and Pierre Curie - (1859-1906)
The discovery of radioactivity by Becquerel in 1896 inspired the Curies in their researches which led to the isolation of polonium, (named after the country of Marie's birth) and radium. Marie concluded that pitchblende contains a small amount of an unknown radiating element. -
Robert Millikan - (22 March 1868 – 19 December 1953)
Millikan worked on an oil-drop experiment and measured the charge on a single electron. He ended up discovering the charge of the electron and helped in the attainment of the electron's mass. They did this by manipulating electrical charges and magnetic fields. -
Niels Bohr - (7 October 1885 – 18 November 1962)
Constructed a model of an atom from his research. Named it the "ground state" aka the lowest allowable energy state of an atom. Introduced the "Bohr Model" even though it wasn't completely correct. It was also known as the planetary model. -
Quantum Model
The quantum theory is the most up-to-date theory of the times. It's model of the atom explains its stability and treats atoms as electrons surrounding a nucleus. This theory gives the location of an electron as a probability. Erwin Schrödinger established the mathematics of quantum mechanics. -
James Chadwick - (October 20, 1891, Manchester UK July 24, 1974, Edinburgh, Scotland)
Proved that the atomic nucleus contained a neutral particle. Even though Ernest Rutherford introduced it a decade earlier. He performed tests on a new type of radiation that scientists had been confused by for years, and this new radiation had also been miscontrued for gamma rays. A sample of Beryllium was bombarded with alpha particles, which causes it to expel this mysterious radiation. -
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