History of the Atom

Democritus

400

All matter consists of invisible particles called atoms.
Atoms are indestructible.
Atoms are solid but invisible.
Atoms are homogenous.
Atoms differ in size, shape, mass, position, and arrangement.
Solids are made of small, pointy atoms.
Liquids are made of large, round atoms.
Oils are made of very fine, small atoms that can easily slip past each other.

John Dalton

Sep 23, 1804

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 rearrangement of atoms

JJ Thomson

Sep 23, 1897

1897- discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube

Robert Milliken

Sep 23, 1908

1908-1917
measured the charge on an electron
by studying individual droplets, he was able to show that the charge was always an integral multiple of a small, but finite value
charge is always some multiple of 1.59 x 10^-19
The mass of an electron is is at least 1000 times smaller than the lightest atom

Neils Bohr

Sep 25, 1913

an electron is found only in circular paths or orbits around the nucleus
an electron's energy is determined on how far away the energy level is from the nucleus
determined how many electrons were allowed on each energy level by giving the energy levels numbers

James Chadwick

Sep 25, 1932

proved the existence of neutrons
neutrons contain no charge

Ernest Rutherford

Sep 23, 2015

The Rutherford model of the atom was simplified in a well known symbol showing electrons circling around the nucleus
1907- Rutherford, Hans Geiger and Ernst Marsden carried out the Geiger-Marsden experiment, an attempt to examine the structure of the atom