Democritus was a integral part of creating atomic theory. He theorized that all material bodies are made up of indivisibly small “atoms.”

Dalton's atomic theory says that everything is made of tiny particles called atoms, and atoms of the same element are exactly the same. It also says that atoms join together in specific ways to make compounds and can't be destroyed in chemical reactions.

J.J. Thomson discovered the electron, showing that atoms are made of smaller parts. He proposed the "plum pudding" model, where atoms are spheres of positive charge with tiny negative electrons scattered inside, like raisins in a pudding.

Robert Millikan conducted the oil drop experiment, which measured the charge of a single electron. His work confirmed that electrons carry a fixed negative charge and helped determine the mass of an electron.

Ernest Rutherford discovered the nucleus through his gold foil experiment. He showed that atoms have a small, dense, positively charged nucleus at the center, with electrons orbiting around it, mostly in empty space.

Niels Bohr proposed that electrons move in specific orbits around the nucleus, each with fixed energy levels. His model explained how electrons jump between orbits, emitting or absorbing energy as light when they do.

Louis de Broglie proposed that electrons have particle and wave properties, meaning they can behave like waves and tiny particles. This wave-particle duality helped explain why electrons occupy specific orbits, as their wave-like behavior affects how they move around the nucleus.