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400 BCE
Democritus
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
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
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
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
The Bohr model postulates that electrons orbit the nucleus at fixed energy levels. Orbits further from the nucleus exist at higher energy levels -
Schrodinger
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.