In antiquity, many opined about "the elements" in combination. Around 2500 years ago, Leucippus and Democritus founded the idea of atoms. Perhaps everything, they opined, was composed of indivisible building blocks. In the late 1700s, hydrogen and oxygen were discovered. Circa 1804, John Dalton revived atomism to explain chemical behavior. Then in 1869, Mendeleev developed the periodic table: organizing the atoms.