Democritus was an ancient Greek philosopher who lived from 460 BC to 370 BC. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated what is thought to be the first atomic theory. Some people consider him to be the father of modern science. It is hard to separate his theories from those of Leucippus, since they are always mentioned in the same texts, but their theories have very different basis.
Democritus claimed that everything is made up of atoms. These atoms are physically, but not geometrically, indivisible; between atoms lies empty space; atoms are indestructible; have always been, and always will be, in motion; there are an infinite number of atoms and kinds of atoms, which differ in shape, and size. He said, about the mass of atoms,”The more any indivisible exceeds, the heavier it is.”. He helped to propose the earliest views on the shapes and connectivity of atoms. He reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling. Using analogies from our senses, he gave an image of an atom that distinguished them from each other by their shape, size, and the arrangement of their parts. These connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets. The Democritean atom is an inert solid that interacts with other atoms mechanically. In contrast, modern, quantum-mechanical atoms interact via electric and magnetic force fields and are far from inert.
He was criticized by many of his contemporaries, including Aristole, because he did not explain the initial cause of the motion of atoms.
There is an article about the Democritus atom. Here on Universe Today we have a couple of great articles about the topic: one about the man himself and the other is about a newer atomic theory. Astronomy Cast offers a good episode about what happens inside the atom.