Who first proposed that matter is composed of tiny, indivisible particles?

When we go back 2000 years to the 19th century, John Dalton proposed a simple atomic theory that attempted to explain the behavior of matter. (If it seems that I am offering disrespect to philosophy, please note that Dalton would have described himself as a natural philosopher!) Democritus was more of a philosopher than an experimental chemist, and none of his writings survive.

According to Dalton's atomic theory, the following are the main points: (i) all elements are made up of small indivisible particles called "atoms" (derived from the Greek word "alphatauomuos," which means "indivisible" or "uncuttable" and may be a nod to ancient Democritus); (ii) all atoms of a given element have the same mass, size, and form; (iii) atoms cannot be created or destroyed; (iv) atoms can combine in simple whole number ratios; and (v) these combinations are examples of chemical reactions.

Modern physicists and chemists would have very little trouble with any of these ideas—and of course, our own idea of atoms and molecules comes from Dalton. Dalton, an accomplished experimentalist, developed his atomic theory largely through experimentation; he was particularly interested in the behavior of gases, which he could study through straightforward experiments.

For historical purposes alone, it is widely conjectured that many of Dalton's ideas were already anticipated by 17th-century polymaths like Galileo and Newton, who were so brilliant that they neglected to record any of their theories regarding atoms.

The ancient Greek philosopher Democritus first proposed the idea that matter is composed of tiny, indivisible particles called atoms.