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456 BCE
Democritus
Democritus's mentor, Leucippis, originally came up with the atomic theory, but it was adopted by Democritus. The theory stated that “The universe is composed of two elements: atoms and the void in which they exist and move. According to Democritus atoms were minuscule quantities of matter, he hypothesized that atoms cannot be destroyed, etc. They are always moving and invisible, this hypothesis was created around 456BC. Democritus’s discovery of atoms was just a theory, no experiment involved.
