Atomic Theory Timeline

Democritus

400

He said the particles that make up matter is an atom.

John Dalton

Sep 25, 1803

He discovered that atoms are spherical solid masses that are in constant motion.
He said that atoms of a given element posses unique characteristics and weight.

X-Rays

Oct 5, 1895

J.J. Thompson's Cathode ray experiment led to farther experimentation that lead to the worlds first x-ray.

J.J. Thompson

Sep 25, 1897

He discovered that cathode rays are negatively charged particles which he called 'corpuscles'.
Also mass was about 1000 times smaller than a hydrogen atom.
He described the atom as a ball of positive charge containing a numnber of Electrons.

Ernest Rutherford

Sep 25, 1909

Used gold foil experiment to determine that the nucleus of an atom os dense and has a positive charge, thus discovering the proton.

Neils Bohr

Sep 28, 1913

He said that the atomic structure was "a successive orbital shell shell of neutrons." He built the Bohr planetary model. This showed how electrons travel in certain orbitals.

Francis Aston

Oct 7, 1919

Francis Aston discovered that for several elements the atomic mass is not the mass of all atoms of the element, therefore discovering what we call the isotope.

Schrodinger

Sep 28, 1920

He identified the electron cloud model as the probable location of many electrons.

James Chadwick

Sep 28, 1932

Chadwick discovered a particle within a nucleous that had a mass similar to the protion. Thus, he discovered the neutron.

Cockroft and Walton

Sep 28, 1932

These two men together split the nucleous of an atom using an accelerated particle accelerator.

Atomic Bomb Dropping

Aug 6, 1949

Two bombs, Little Boy and Fat Man, were dropped on Hiroshima and Nagasaki Japan to end World War 2.

Isotopes Used In Medicine

Oct 5, 1972

Aston's discovery of isotopes led to them being used in CAT scans.