Democritus, the ancient Greek philosopher, is often credited with the early concept of the atom as the smallest indivisible unit of matter. He theorized that atoms were tiny, indivisible particles that made up all matter in the universe.
Democritus envisioned atoms as having various shapes and sizes, which would determine the properties of the substances they formed. For example, he believed that atoms of different elements could have different shapes—some could be smooth and round, while others could be rough and jagged. This variety in shape and size would account for the differences in physical properties observed in various materials.
Overall, although Democritus did not have the tools or knowledge of modern atomic theory, his ideas laid the groundwork for later scientific exploration of the nature of matter. His concept of atoms being the fundamental building blocks of matter is foundational to the development of chemistry and physics.