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Period: 300 to
History of the Atomic Theory
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450
Demoritus 370 BC - 460 BC
His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.” -
John Dalton 1766 - 1844
All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical in mass and properties. Compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms. Atoms can be neither created nor destroyed. -
J.J Thomson 1856 - 1940
An atom consists of a sphere of positive charge with negatively charged electron embedded in it. The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge. -
Ernest Rutherford
He showed that while the nucleus contains virtually all of the mass of the atom, it only takes up one-billionth of the volume of the atom, an inconceivably tiny amount. Much smaller particles - electrons - orbit the nucleus at a great distance, relatively speaking. -
Niels Bohr 1885 - 1962
proposed that electrons could orbit the nucleus without losing energy; could move only in fixed orbits of specific energies. -
James Chadwick 1891 - 1974
proved the exsistence of neutrons
