jan 1, 442 BC - Democritus' Atomic Theory
Democritus' theory was discovered in 442 BC. Through observations, he concluded that if you keep dividing matter in half, you will finally result with an "atomos." Atomos means that the particle can't be broken down any farther. Below is a picture of an atom being divided into smaller atoms.
Added to timeline:
Atomic Theory Timeline - Adrianne Moran 4th Period Chemistry