c.430 bc

Greek natural philosopher Empedocles (d. c. 430 bc) proposes that all matter consists of four elements: earth, air, fire, and water.

c.400 bc

Greek natural philosopher Democritus of Abdera (c. 460–370 bc) proposes that all matter consists of atoms.

c.306 bc

Greek philosopher Epicurus (c. 342–270 bc) champions Democritus’ atomic theory.

1649

French philosopher Pierre Gassendi (1592–1655) proposes an atomic theory (having read Epicurus).

1803

John Dalton proposes Dalton’s atomic theory.

1897

J. J. Thomson discovers the electron.

1904

J. J. Thomson proposes his ‘plum pudding’ model of the atom, with electrons embedded in a nucleus of positive charges.

Japanese physicist Hantaro Nagaoka (1865–1950) proposes a ‘Saturn’ model of the atom with a central nucleus having a ring of many electrons.

1911

New Zealand-born physicist Ernest Rutherford (1871–1937) discovers the atomic nucleus.

1913

Niels Bohr proposes model of the atom with a central nucleus surrounded by orbiting electrons.

British physicist Henry Moseley (1887–1915) equates the positive charge on the nucleus with its atomic number.

Frederick Soddy discovers isotopes.

1916

German physicist Arnold Sommerfield (1868–1951) modifies Bohr’s model of the atom specifying elliptical orbits for the electrons.

1919

Ernest Rutherford discovers the proton.

1920

Ernest Rutherford postulates the existence of the neutron.

1925

Werner Heisenberg discovers matrix mechanics.

Wolfgang Pauli postulates the exclusion principle.

1926

Erwin Schrödinger discovers wave mechanics.

1932

British physicist James Chadwick (1891–1974) discovers the neutron.

Werner Heisenberg proposes a model of the atomic nucleus in which protons and neutrons exchange electrons to achieve stability.

1937

Niels Bohr proposes a ‘liquid drop’ model of the atomic nucleus.

1948

German-born US physicist Marie Goeppert-Meyer (1906–72) and German physicist Hans Jensen (1907–73) independently propose the ’shell’ structure of the nucleus.

1950–53

Danish physicist Aage Bohr (1922–2009), US physicist Ben Mottelson (1926– ), and US physicist Leo Rainwater (1917–86) combine the ‘liquid-drop’ and ’shell’ models of the nucleus into a single theory.