Timeline

Democritus

400 BCE

Democritus thought that atoms are tiny, uncuttable, solid particles that are surrounded by space and constantly moving at random. Democritus surmised that different kinds of matter consist of different types or arrangements of atoms.

John Dalton

1803

Dalton's atomic theory suggested that all matter was comprised of indivisible and indestructible atoms with distinct masses and properties, the combination of which determined the physical nature of their constituent elements.

J.J. Thomson

1897

Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

Rutherford

1911

Rutherford described the atom as having a tiny, dense, and positively charged core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus. The light, negatively charged, electrons circulated this nucleus, much like planets revolving around the Sun.

Bohr

1913

The Bohr model postulates that electrons orbit the nucleus at fixed energy levels. Orbits further from the nucleus exist at higher energy levels

Schrodinger

1921

could be regarded as both particles and waves, in 1926 Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms.