HKittel's History of the Atomic Theory

Democritus

Oct 26, 1000

Democritus
Existed in 500 B.C.
Democritus believed that all matter consisted of extremely small particles that could not be divided.

Aristotle

Oct 27, 1000

Existed in 500 B.C.
Aristotle did not think there was a limit to the number of times matter could be divided.
He described matter by using a model.

George Hadley

Oct 31, 1685

George Hadley's Life
Hadley was an English lawyer and amateur meteorologist who proposed the atmospheric mechanism by which Trade Winds are sustained.

John Dalton

Oct 27, 1766

Dalton pictures atoms as tiny, indestructible particals, with no internal structure.
Dalton gathered evidence for the existence of atoms by measuring the masses of elements that combine when compounds form.

Henry Moseley

Oct 31, 1887

A British chemist, studied under Rutherford and brilliantly developed the application of x-ray spectra to study atomic structure.
Moseley's discoveries resulted in more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers.

J.J. Thomson

Oct 27, 1897

A British scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a shere of positive electric charge.
Thomson used a device. At the center of the device is a sealed glass tube from which most of the air has been removed. There is a metal disk at each end of the tube. Wires connect the metal disks to a source of electric current. When the current is turned on, one disk becomes negitively charged and the other disk becomes positively charged.

Hantaro Nagaoka

Oct 27, 1904

Hantaro
A Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around saturn.

Ernest Marsden

Oct 28, 1909

Marsden could figure out the path of an alpha particles after it passed through gold.
Marsden aimed a narrow beam of alpha particles at the gold. The screen around the gold was made of a material that produced a flash of light when struck by a fast-moving alpha particle.

Ernest Rutherford

Oct 27, 1911

Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus.
Marsden and Rutherford both worked together on The Gold Foil experiment.

Niels Bohr

Oct 27, 1913

Niel Bohr
Bohr says that the electrons move in a spherical orbits at fixed distances from the nucleus.

Louis de Broglie

Oct 28, 1924

broglieBroglie proposes that moving particles like electrons have some properties of waves.

Erwin Schrodinger

Oct 27, 1926

Schrodinger describes the motion of electrons in atoms. His work leads to the electron cloud model.
Develops mathematical equations.

James Chadwick

Oct 27, 1932

ChadwickA British physicist, confirms the existence of neutrons which have no charge. Atomic nuclei contain neutrons and positively charged protons.