The idea of the atom was originally proposed by ancient scholars. The philosophers Democritus and Leucippus are often credited with proposing the early notions of the atom, including the ideas that many different kinds of atoms exist, that there is a substantial amount of empty space between atoms, and that their properties are responsible for the properties of materials we see and interact with. For centuries, ideas about the structure and properties of the atom were based largely on conjecture and logical arguments, and it wasn’t until the 1800s that experiments began to allow atomic theory to advance to where it is today.