Atomic Theory Timeline

Events

Democritus

Approx. 400 B.C.

Stated that all matter is made up of matter and also stated that atoms are eternal and invisible and so small that they cannot be divided and completely fill up the space they are in

Aristotle

Approx. 300 B.C.

Believed matter to be comprised of fire, air, earth, and water rather than small particles; caused Democritus’s ideas to be overlooked for centurie

Isaac Newton

1704

Proposed a mechanical universe with small solid masses in motion

Antoine Lavoisier

1777

Provided formula for conservation of mass and distinguished elements from compounds

Joseph Proust

1794

Discovered the law of constant composition

John Dalton

1803

Formed the atomic theory, stating that all matter is composed of tiny indestructible particles called atoms that are all alike and have the same atomic weight

Amadeo Avogadro

1811

Hypothesized that two given samples of an ideal gas, at the same volume and at the same temperature and pressure, contain the same number of molecules

J.J. Thompson

1898

Discovered the electron and determined its charge to mass ratio (1.759 x 108 coulombs/gram) and developed the plum pudding model

Marie Curie

1898

Studied uranium and thorium and called their spontaneous decay process “radioactivity;” also discovered polonium and radium

Max Planck

1900

Developed the quantum theory

Albert Einstein

1905

Postulated that light was made up of different particles that, in addition to wavelike behavior, demonstrated certain properties unique to particles; developed the theory of relativity (E=mc2)

Robert Millikan

1908

Through the oil drop experiment determined the charge (1.602 x 10-19 coulombs) and the mass (9.11 x 10-28 grams) of an electron

Ernest Rutherford

1909

Used results of the gold foil experiment to state that the mass of an atom was in a small positively-charged sphere at the center of an atom

Niels Bohr

1913

Stated that electrons moved around the atom in successively large orbits; states that atoms absorb or emit radiation only when the electrons abruptly jump between allowed, or stationary, states; developed an explanation of atomic structure that underlies regularities of the periodic table of elements