History of the Atomic Theory

Democritus and Aristotle

460 BCE

Democritus, a Greek philosopher, believed that matter was made up of tiny indivisible and indestructible particles that he called "atomos". His theory was not accepted because it lacked experimental support, and Aristotle disagreed, saying that all things were composed of earth, air, fire, and water.

John Dalton

Oct 21, 1803

Dalton was an English chemist and school teacher. He used Lavousier's scientific method while working with gases to form his atomic theory.
Dalton's Atomic Theory:
1. All elements are composed of tiny indivisible particles called atoms
2. Atoms of the same element are identical
3. Atoms of different elements can mix together physically or combine chemically to create compounds
4. Chemical reactions can join, separate or rearrange elements in a compound

Eugen Goldstein

Jan 1, 1886

Goldstein was born in Poland, but is known as a German physicist. His work with Cathode Ray Tubes suggested the existence of the proton. He won the Hughes Medal in 1908.

J.J. Thomson

Apr 30, 1897

Thomson was an English physicist. He performed experiments with a Cathode Ray Tube (invented by Crookes) to discover the electron in 1897. A few years later, in March of 1904, Thomson introduced the first model of the electron, called the "plum pudding model."

"Plum Pudding Model"

Mar 1, 1904

Intoduced by J.J. Thomson in March of 1904. In this model the "pudding" represents positively chaged material in which the negatively charged "plums/raisins" reside.

Ernest Rutherford

May 1, 1911

Rutherford was born in New Zealand, but is known as an English chemist. His contribution to the atomic theory was the result of the Gold Foil experiment performed with Geiger and Marsden. In their experiment, the men shot alpha particles at a sheet of gold foil. They observed that some particles bounced straight back rather than passing through with a slight deflection. As a result, the idea of a positively charged nucleus was born.
https://www.youtube.com/watch?v=5pZj0u_XMbc

Niels Bohr

Jul 1, 1913

Bohr was a Danish physicist and student of Ernest Rutherford. Following the discovery of the nucleus, he developed the Nuclear Atom model. In this model the electrons orbit the nucleus and occupy most of the volume of the atom. The Nuclear Atom Model is referred to as "planets around the sun." Bohr discovered the energy levels of electron orbitals, and also developed the Quantum Model of the atom. Niels Bohr recieved the Nobel Prize in Physics in 1922.

Robert Millikan

Aug 1, 1913

Millikan was a physicist from the United States. He measured the charge of an electron and determined that the charges of all electrons are the same by using his "falling-drop method." He published his original work in 1910, but published a new and improved version in August of 1913 following the dispution of his findings. Robert Millikan won the Nobel Prize in Physics in 1923.

Erwin Shrodinger

Dec 1, 1926

Shrodinger was an Austrian physicist. He used the results of theoretical equations to devise and solve mathematical equations that describe the behavior of atoms. His results still restricted atoms to energy levels, but showed that there was no exact path on which they traveled. His Quantum Mechanical Model introduced the idea of an electron cloud. Schrodinger won the Nobel Prize in Physics in 1933 and a crater on the dark side of the moon is named after him.

James Chadwick

May 1, 1932

Chadwick was an English physicist. He proved the existence of neutrons, which are subatomic particles with no charge. Chadwick contributed to the fission of uranium 235 and towards the creation of the atomic bomb. For this discovery, he was awarded the Hughes Medal of the Royal Society in 1932, and the Nobel Prize for Physics in 1935.