Who proposed that all matter is composed of tiny particles called atoms?

The proposition that all matter is composed of tiny particles called atoms can be traced back to the ancient philosopher Democritus. He was among the first to suggest that everything in the universe is made up of indivisible particles, which he named "atomos," meaning "indivisible." His ideas laid the groundwork for later scientific developments, even though they were not based on experimental evidence, as modern atomic theory would be.

Democritus's vision was philosophical and speculative, as he lacked the tools and methodologies of modern science. It wasn’t until centuries later, particularly with the work of John Dalton, that the atomic theory was formalized into a scientific framework supported by experimental data. Dalton built upon such ideas to formulate his own atomic theory, which included a more structured understanding of atoms and their combinations in chemical reactions.

In contrast to the other figures mentioned, JJ Thomson contributed to atomic theory by discovering the electron, and Erwin Schrödinger developed quantum mechanical models of atoms, but neither proposed the original concept that all matter consists of atoms as fundamental particles.