Timeline of Atomic Theory Contributions

By: Jasmine Parsons



332 BCE

Aristotle believed that all materials on earth was not made of atoms, rath-er a combination of fire, water, air and earth.


322 BCE

Democritus hypothesized that the world was made up of particles called atoms. He believed they were indestructible, indivisible, and always in m-otion.

Robert Boyle


Boyle discovered that as volume increases, the pressure of the gas decr-eases in proportion. Similarly, as volume decreases, the pressure of the -gas increases which in sum is what is known as Boyle's Law.

John Priestley


Priestley discovered "dephlogisticated air", which is known as oxygen.

Antoine Lavoisier


He explained combustion and proposed the law of conservation of mass, which was the bases for Dalton's work. His started the conversation of w-hat an atom really was.

Joseph Proust


Proust discovered and stated the Law of Definite Proportions, which allo-wed for the set up of Dalton's theory,

John Dalton


John Dalton's was the first to come up with atomic theory with a four part theory. He believed atoms were invisible particles we couldn't see. He cr-eated a basis for modern atomic theory.

William Crookes


Discovered cathode rays ravel in straight lines from the cathode; impart a negative charge to objects they hit; are deflected by electric fields and m-agnets to suggest a negative charge; cause glass to fluoresce; cause pinwheels in their path to spin showing they have mass.

JJ Thompson


Thomson tested the polarity of atoms and found almost all particles had a negative charge. He also developed the plum pudding model of an atom. He even discovered electrons which were the first subatomic particles to be identified.

Marie Curie


Marie along with Pierre studied radioactive materials. They studied urani-um and thorium and saw a decay process which they named radioactivity.

Pierre Curie


While working on radioactive substances, Pierre discovered gamma rays. He also studied with his wife Marie to discover radioactive elements of p-olonium and radium.

Jean Baptiste Perrin


Perrin Supported Albert Einstein's theory that Brownian motion, the rand-om movement of small particles in a liquid, was because of collisions bet-ween the particles and molecules in the liquid.

Hans Geiger


Geiger developed an electrical device, the Geiger Müller tube that clicks when hit with alpha particles.

Robert Millikan


Millikan performed the oil drop experiment which determined the charge (e=1.602 x 10 -19 coulomb) and mass (m = 9.11 x 10 -28 gram) of electrons.

Ernest Rutherford

1911 - 1917

He came to the conclusion alpha particles are positive, beta particles are negative, and neutral a neutral particle is light.

James Chadwick


James Chadwick showed that a nucleus contains neutrons along with pr-otons.