Atomic Theory



500 B.C - 1720

Practiced trying to change non-precious metals into gold and silver.

Democritus of Adbera

450 B.C

Philosophized atomos are the building blocks of matter. Atoms of the same element were the same and atoms of different atoms were different. Characteristics of the the atoms were determined by its shape and atoms remained unchanged when combined with other atoms.


335 B.C

Proposed all matter consisted of four elements: fire, water, earth, and air. These elements consisted of four qualities: dryness, wetness, coldness, and hotness. Aristotle's theory was believed for 2,000 years.

Benjamin Franklin


Experimented with electricity and proved electrons had an electrical charge.

Antoine Lavoisier


The French chemist discovered the Law of Conservation of Mass. This law states matter can neither be created nor destroyed.

Joesph Proust


Proust, like Lavoisier, was also a French chemist. Proust discovered the Law of Constant Composition which states given compounds always contain the same elements in the same proportions.

John Dalton


In 1803 Dalton proposed the Atomic Theory. There were four parts: 1) All matter is composed of extremely small particles called atoms. 2) Atoms of a given element are the same size, mass, and other properties. 3) Atoms cannot be subdivided or destroyed 4) Atoms of different elements can be combined to form simple compounds. 5) In chemical reactions, atoms are combined and/or rearranged and/or separated.

Michael Faraday


Developed laws of electrolysis in 1832. The laws state that electrolysis is the splitting of molecules with electricity.

Dmitri Mendeleev


Developed Periodic Law which states elements are arranged into 7 groups with similar properties. Elements are "periodic functions of their atomic weight."

William Crookes


William Crookes studied cathode rays. Through his observations he was able to conclude that cathode rays: travel in straight lines, impart a negative charge on objects they strike, are deflected by electric fields and negative sided magnets, and also cause pinwheels to spin. Crookes was also able to conclude that cathode rays have mass because they can make a pinwheel move.

Henry Becquerel


Henry Becquerel discovered radioactivity and radiation from some elements.