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Atomic theory Timeline
Timeline Description: Atomic theory is the scientific theory of the nature of matter. The theory states that matter is made up of small particles called atoms. Prior to this theory, matter was thought to be able to be divided into any small quantity. The word atom is derived from the Greek atmos, meaning indivisible.

Date Event
442 BC Thinking about matter

Democritus, a Greek philosopher, co-originated the thought (with his teacher, Leucippus) that all matter is composed of indivisible elements.
1803 John Dalton and atoms

John Dalton, a British chemist and physicist, developed a theory that matter is simply composed of atoms of different weights and is combined in ratios by weight. Also proposed that these atoms are spherical, and are in motion.
1870 Cathode ray tube and TV (1870's)

Sir William Crookes constructed a primitive cathode ray tube, which later became the basis for television.
1896 X-rays developed

Wilhelm Rontgen discovered that certain chemicals glowed when exposed to cathode rays. These rays weren't deflected by a magnetic field produced in the cathode ray tube. He named these X-rays.
1898 Radiation, energy, and the atom

Pierre and Marie Curie theorized that radioactive particles cause atoms to break down, then releasing radiation that takes the form of energy and subatomic particles.
1898 Electrons discovered

JJ Thomson discovers the electron, using properties of cathode rays.
1900 Early quantum theory developed

Max Planck introduced what would be known as quantum theory, stating that electromagnetic energy could only be emitted in quantized form. His quanta are now called photons by physicists.
1905 Einstein and the nature of light

Albert Einstein, creates special and general theories of relativity, and hypothesizes about the particle nature of light. This was the basis of nuclear energy.
1908 Charge of an electron measured (1908-1917)

Robert Millikan measured the charge of a single electron. This is known as the elementary charge, one of the fundamental physical constants.
1909- Structure of an atom developed (1909-1911)

Ernest Rutherford known as the father of nuclear physics, developed the theory for the structure of the atom. He used a gold foil experiment, observing the scattering of alpha particles, and demonstrated for the first time the existence of the atomic nucleus.
1913 Bohr improves the atomic model

Neils Bohr developed the Bohr atomic model, with electrons travelling in orbits around the nucleus, and chemical properties being determined by how many electrons are in the outer orbits. He also integrated the Planck quantum theory, stating that when electrons change orbits they emit a quantum of discrete energy.
1926 Math describes electron changes

Erwin Shrodinger described how electrons move in wave form, and developed the Schrodinger equation which describes how the quantum state of a system changes with time.
1931 The neutron is discovered

James Chadwick discovers the neutron component of the atomic nucleus, explaining the nuclear fission of uranium 235. This also made it possible to produce elements heavier than uranium in the lab.
1938 Nuclear fission developed

Otto Hahn, regarded as the father of nuclear chemistry, discovers nuclear fission, along with Lise Meitner.
1951 Nuclear medicine and I-131

Glenn Seaborg, many discoveries of the transuranium elements, as well as many advances in nuclear medicine, including the development of I-131 for thyroid disease.
1964 Elementary particles smaller than the atom Murray Gell

Mann proposes the quark model (independently George Zweig does as well), which describes elementary particles that have no substructure (and therefore can't be split).