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400 BCE
Democritus/ Dalton’s model
The theory that Democritus developed was that everything is composed of atoms, which are physically, but no geometrically, indivisible. -
Thomson's Model
The experiments of J.J. Thomson with the cathode ray tube showed that all atoms contain microscopic, negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which has negatively charged electrons embedded in a positively charged "soup". -
Rutherford's Model
Ernest Rutherford discovered that the atom was mostly empty space, with almost all of its mass concentrated in a tiny central nucleus. The nucleus has a positive charge and is surrounded at great distances by negatively charged electrons.
In 1919 Rutherford had discovered the proton, a positively charged particle within the atom's nucleus. -
Bohr's Model
The discovery of electrons and radioactivity in the late 19th century. In 1913, Niels Bohr proposed a theory for the hydrogen atom, based on the quantum theory that certain physical quantities take only discrete values. Electrons move around a nucleus, but only in specified orbits, and if the electrons move to orbits of lower energy. Bohr's model explains why the atom emits only light of a fixed wavelength, and then incorporates theories of light quanta. -
Quantum Mechanical Model
The quantum mechanical model of the atom comes from the solution of the Schrödinger equation. Quantifying the energies of electrons is a requirement for solving equations. This is different from Bohr's model, where quantization is simply assumed without any mathematical basis. The physicists Werner Heisenberg, Max Born, and Pascual developed matrix mechanics while Schrödinger invented wave mechanics. -
Protons, Neutrons, & Electrons
Rutherford had discovered the proton, a positively charged particle within the atom's nucleus. In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons – elementary particles devoid of any electrical charge. Although J.J. Thomson is credited with the discovery of the electron on the basis of his experiments with cathode rays in 1897, various physicists, including William Crookes, Arthur Schuster, Philipp Lenard, and others.