The History of the Atom

Democritus

442 BCE

Democritus discovered that everything is made up of atoms, and that atoms are invisible. He also named the atom because atomos means uncut, and he said that the shape of the atom corresponds to the item it is making up.

John Dalton

1803

John Dalton came up with four main ideas of atomic theory. They were:
1. All matter is made up of atoms.
2. All atoms of an element have a fixed mass.
3. Compounds contain atoms from different elements.
4. In compounds, atoms from different elements always
combine in a specific way.

J.J. Thompson

1897

J.J. Thompson discovered the very important electron. He also said that matter in general had a positive charge, and that atoms were organized like plum pudding.

Marie Curie

1898

Marie Curie discovered both radium as a pure metal and polonium.

E. Goldstein

1900

Eugen Goldstein discovered the proton and found that it has a positive charge. He also said that atoms need to be electrically neutral.

Ernest Rutherford

1909

Ernest Rutherford discovered that atoms contain a nucleus, and he said that atoms are made up of two parts, the nucleus and the extranuclear part.

Robert Millikan

1910

Robert Millikan discovered that electrons have a negative charge.

Niels Bohr

1913

Niels Bohr came up with the planetary structure of the atom. This model has the nucleus in the center of the atom with protons and neutrons, and then the electrons revolve around the nucleus in different imaginary paths called orbits or shells.

James Chadwick

1923

James Chadwick discovered the neutron and found that it has no charge. He also discovered that neutrons make up the nucleus along with protons.

Quantum Mechanical Model

1926

The Quantum Mechanical Model is based on quantum theory, and it is a model where electrons are usually located in an electron cloud instead of specific orbital paths.