Proposed that all matter is made up of tiny, indivisible particles called "atomos."

Believed atoms differed in shape and size but were otherwise unchangeable.

Rejected Democritus' idea of atoms.

Believed matter was made of four elements: Earth, Water, Air, and Fire.

Proposed the first modern atomic theory.

Stated that atoms are indivisible and combine in whole-number ratios to form compounds.

Discovered the electron using the cathode ray tube experiment.

Proposed the "Plum Pudding Model," where negatively charged electrons were embedded in a positively charged sphere.

Conducted the gold foil experiment.

Discovered that atoms have a dense, positively charged nucleus surrounded by empty space.

Developed the quantum mechanical model.

Described electron locations as probability clouds rather than fixed orbits.