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500 BCE
Democritus
Concluded that everything in the world consists of atoms, which are indestructible, invisible, always in motion, and infinite in number and size/shape. -
400 BCE
Aristotle
Noted that the most ordinary, material things are composed of multiple substances. Hypothesized about heterogeneous and homogeneous mixtures. -
Antoine Lavoisier
Found that mass is conserved in a chemical reaction, and that the total mass of the products of a chemical reaction is always the same as the total mass of the starting materials. Results have led to the law of conservation of matter. -
John Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. -
J.J. Thomson
Created the Thomson model, or "plum-pudding" model, in which negatively charged electrons (plums) were mixed with smeared-out positive charges. -
Robert Millikan
Accurately determined the charge and mass of an electron, and proved that this quantity was a constant for all electrons. -
Ernest Rutherford
Through his gold foil experiment, he established that the nucleus was very dense, small, and positively charged. Assumed that electrons are located outside the nucleus. -
Niels Bohr
A student of Rutherford's who developed the Bohr model, which has electrons arranged in concentric circular orbits around the nucleus and don't lose energy doing so. -
Louis de Brogile
Discovered that electrons had a duel nature, similar to both particles and waves. Suggested that all matter has wave properties and began studying the quantum theory. -
Erwin Schrodinger
Took the Bohr atom model one step further, using mathematical equations to describe the likelihood of finding an electron in a specific location, known as the quantum mechanical model of the atom. -
Werner Heisenberg
Proposed the Principle of Indeterminancy - you cannot know both the position and the velocity of a particle. -
James Chadwick
Using alpha particles, discovered a neutral atomic particle with a mass close to a proton - the neutron.