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.

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.

1903

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

Ernest Rutherford discovers the atomic nucleus.

1913

Niels Bohr proposes model of the atom with a central nucleus surrounded by orbiting electrons and the orbits characterized by quantum numbers.

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

Frederick Soddy discovers isotopes.

1916

German physicist Arnold Sommerfeld (1868–1951) modifies Bohr’s model of the atom, specifying elliptical orbits for the electrons; introduces azimuthal and magnetic quantum numbers in addition to the principal quantum number (see atom).

1919

Ernest Rutherford achieves the first artificial transmutation of elements.

1920

Ernest Rutherford postulates the existence of the neutron.

1925

Wolfgang Pauli proposes a fourth quantum number, subsequently identified as spin, and proposes his exclusion principle.

Werner Heisenberg formulates quantum mechanics in terms of matrices.

1926

Erwin Schrödinger formulates quantum mechanics in terms of waves.

1932

James Chadwick discovers the neutron.

Werner Heisenberg proposes a model of the atomic nucleus in terms of protons and neutrons.

1936

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

1948

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

Early 1950s

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