History of the Atom

Aristotle

322

proposed that matter was continuous and was not made up of smaller particles. He called the continuous substace "hyle"

Democritus

400

He suggested that the world was made of two things--empty space and tiny particles called "atoms." He thought that the atoms were the smallest possible particles of matter and that there were different types of atoms for each material.

Isaac Newton

Oct 18, 1704

Proposed a mechanical universe with small solid masses in motion

Antoine Lavoisier

Jun 15, 1743

found that when a chemical change occured in a closed system, the mass after a chemical change equaled the mass before the change. He proposed that in ordinary chemical reactions matter can be changed in many ways but it cannot be created or destroyed.

Joseph Proust

Jul 15, 1799

proposed the the Law of Definite Proportions. He observed that specifc substances alwasy contain elements in the same ratio by mass.

John Dalton

Jul 16, 1803

proposed the Law of Multiple Proportions. \Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. He also developed the concept of the mole and proposed a system of symbols to represent atoms of different elements. He developed a relative atomic weight scale from percent composition data and assumed atomic ratios

J. Plucker

Jul 13, 1859

Built one of the first gas discharge tubes ("cathode ray tube").

G.J. Stoney

Mar 17, 1894

Proposed that electricity was made of discrete negative particles he called electrons

J. J. Thomson

Aug 16, 1897

identified the negatively charged electron in the cathode ray tube. He figured out that the electron was a component of all matter and calculated the charge to mass ratio for the electron.

Robert Millikan

May 17, 1909

determined the unit charge of the electron with his oil drop experiment at the University of Chicago, allowing for the calculation of the mass of the electron and the positively charged atoms.

Ernst Rutherford

Sep 14, 1911

proposed the nuclear atom as the result of the gold-foil experiment. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons.

Francis Aston

May 13, 1920

invented the mass spectrograph. He was the first person to observe isotopes. He served that there were three different kinds of hydrogen atoms. While most of the atoms had a mass number of 1 he also observed hydrogen atoms with mass numbers of 2 and 3.

James Chadwick

Nov 11, 1932

discovered the neutron.... discovery of the neutron led directly to the discovery of fission and ultimately to the atomic bomb.