History of the Atom and Evolution of the Atomic Model

Democritus

400 BCE

He was the first person to identify the possibility of an atom. He described an indivisible, sphere, solid model.

Aristotle

300 BCE

Aristotle dismissed Democritus’ atom as “worthless.” Said that the four elements were earth, fire, air and water.

John Dalton

Dec 6, 1803

Father of modern atomic theory; five parts of his atomic theory including that elements are composed of atoms and that element’s atoms are identical in mass.

J.J. Thompson

Dec 6, 1897

Discovered the electron. He also designed the Plum Pudding model.

R.A. Millikan

Dec 6, 1909

Determined the charge and mass of an electron through the oil drop experiment.

Ernest Rutherford

Dec 6, 1911

Said that an atom has a small positive nucleus and the remainder is empty space through the gold foil experiment. He designed a model which is called the Nuclear model.

Henry Moseley

Dec 6, 1914

He stated that the atomic number of an element is equal to the number of protons in the nucleus. Designed the periodic table according to atomic mass.

Niels Bohr

Dec 6, 1922

Predicted that electrons travel in specified energy levels. He described a model which looks like the planets in the solar system called Planetary model.

Victor Louis de Broglie

Dec 6, 1923

Worked with Einstein to determine that electrons were similar to particles and waves.

Werner Heisenberg

Dec 6, 1927

Came up with the “uncertainty principle”: It is impossible to determine the position and momentum of a particle at the same time. Finalised the Quantum Mechanical Model.

Erwin Schrodinger

Dec 6, 1930

Viewed electrons as a cloud and introduced wave mechanics as a model for the atom.

James Chadwick

Dec 6, 1932

Discovered the neutron.