Who said that atoms constantly move?

Democritus first stated that atoms constantly move in the fifth century. He and another Greek philosopher, Leucippus, created the concept in conjunction with the greater theory of atomism.

The theory suggested that the mobility of atoms explained change within the universe. It also stated that atomic movement is in relation to its mass and that atoms exist in different sizes. The development of atomism and the notion of perpetual atomic motion was in reaction to the assertion of a Greek philosopher, Parmenides, of the impossibility of change of matter in the universe. Democritus countered that the universe consists of atoms and nothingness within which the atoms move.