Atomic Theory



0460 bc - 0370 bc

Democritus was an ancient Greek philosopher that believed all matter is consisted of extremely small particles. He called them atoms which came from the Greek word atomos which means indivisible.


0384 bc - 0322 bc

Believed all matter consisted of the five elements, water, fire, air, and earth.

Issac Newton

1704 ad

Proposed a mechanical universe with small masses in motion.

John Dalton

1776 ad - 1844 ad

Solidified the atomic theory. He proved the existence of atoms with experiments and came to the conclusion that all elements are made of submicroscopic parts called atoms.

Micheal Faraday

1791 - 1867

Studied the effect of electricity on solutions made the term electrolysis and used it to split molecules.

G.J. Stoney

1826 - 1911

Proposed that electricity was made of negative particles called electrons.

James Clerk Maxwell

1831 - 1879

Proposed electromagnetic fields.

Sir William Crookes

1832 - 1919

Discovered that cathode rays have a negative charge, travel in straight lines, and cause glass to fluoresce. They caused pin wheels to spin indicating they have mass.

Demitri Mendeleev

1834 - 1907

Created the first version of the periodic table of elements.

Wilhelm Roentegen

1845 - 1923

Using a CRT he discovered that nearby chemicals glowed. Further experimentation showed that they were penetrating rays he named X-rays.

Henry Becquerel

1852 - 1908

While studding the effects of X-rays on photographic film, he discovered that some chemicals spontaneously decompose and give off penetrating rays.

J.J. Thomson

1856 - 1940

Used a CRT to experimentally determine the charge to mass ratio e/m of an electron 1.759 x 10 8 coulombs/gram. Studied canal rays and found the were associated with the proton H+.

Max Planck

1858 - 1947

Used the idea of quanta to explain glowing hot matter.

J. Plucker

1859 - 1860

Built one of the first cathode ray tubes.

Niels Bohr

1862 - 1962

Created the atomic model with orbital shells of electrons using the regularities of the periodic table.


1865 - 1950

Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

Marie Skłodowska-Curie

1867 - 1934

Studied uranium and thorium and called their decay process radio activity. Also discovered the radioactive elements polonium and radium.

R.A. Milikan

1868 - 1953

Oil drop experiment determined the charge and mass of an electron.


1869 - 1910

Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity. Died crashing a gas balloon.

Ernest Rutherford

1871 - 1937

Using alpha particles as bullets, probed the atoms in a piece of thin gold foil. He established that the nucleus was: very dense, very small and positively charged. He also assumed that the electrons were located outside the nucleus.


1871 - 1937

Studied radiation emitted from uranium and thorium and named them alpha and beta.


1877 - 1956

Observed spontaneous disintegration of radioactive elements he called isotopes or new elements.

Lise meitner, Hahn, Strassman

1878 - 1968

Discovered fission chain reaction by using heavy elements to capture neutrons and form unstable products.

Albert Einstein

1879 - 1955

E=mc2. Possibly the smartest man that has ever lived.

Hans Geiger

1882 - 1945

Developed an electrical device to click when hit with alpha particles. Created the Geiger counter.

E. Goldstein

1886 - 1887

Used a CRT to study canal rays which had electric and magnetic properties opposite of an electron.

H.G.J. Moseley

1887 - 1915

Using x-ray tubes, determined the charges on the nuclei of most atoms. This work was used to base the periodic table off of atomic number instead of mass


1887 - 1961

Viewed electrons as clouds and introduced wave mechanics as a model of the atom.

James Chadwick (Jimmy Neutron)

1891 - 1974

Discovered a neutral atomic particle using alpha particles with a mass close to a proton, thus the neutron was born!

De Broglie

1892 - 1987

Discovered electrons have a dual nature, similar to both particles and waves.


1897 - 1995

Built an early linear accelerator with protons to produce alpha particles.


1901 - 1976

Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminacy.

Enrico Fermi

1901 - 1954

Controlled the first chain reaction by releasing energy from an atoms nucleus.

Paul Dirac

1902 - 1984

Discovered anti-particles. Discovered the anti-electron in 1932.

Glenn Seaborg

1912 - 1999

Synthesized 6 trans uranium elements and suggested a change in the layout of the periodic table. American scientist who won the 1951 Nobel Prize in Chemistry for discoveries in the chemistry of the trans uranium elements


1919 - 1920

Discovered the existence of isotopes using a mass spectrograph.