Atomic Theory Timeline

Democritis

450

Of course he is a very important scientist since he was the first man who discovered this theory and also thought that all matter was made up of atoms and they could not be destroyed or erased

John Dalton

Sep 6, 1803

Same two elements can combine in different compounds.Matter is composed of tiny indivisible particles called atoms.
All atoms of the one element are identical but are different from the atoms of other elements.
Chemical reactions consist of rearranging atoms in simple whole number ratios.

Joseph J Thomson

Apr 30, 1904

"Plum Pudding" model of the divisible atom:
Atoms consist of a large sphere of uniform positive charge embedded with smaller negatively charged particles (corpuscles).
The total positive charge of the sphere equals the total negative charge of the corpuscles.

Ernest Rutherford

May 11, 1911

Proposed a nuclear model of the atom in which:
a very small positively charged nucleus containing most of the mass of the atom
a very large volume around the nucleus in which electrons move
a nucleus containing positively charged protons
a number of protons equal to the number of electrons

Niels Bohr

Feb 10, 1913

Proposed a 'planetary' model for the hydrogen atom:
Electrons move around the nucleus in fixed orbits (like planets around the sun). An electron in a particular orbit has constant energy.
An electron can absorb energy and move to a higher energy orbit of larger radius. (excited electrons)
An excited electron can fall back to its original orbit by emitting energy as radiation.
Electrons can only exist in certain discrete energy levels

James Chadwick

Nov 12, 1932

The nucleus of an atom contains neutrons, electrically neutral particles with a mass similar to that of a proton. Bombarded beryllium with alpha particles and discovered Rutherford's missing neutral particles.
The discovery of neutrons explained the existence of isotopes, first observed in 1920 by Francis Aston when he invented the mass spectrograph.