Who ﬁrst proposed that matter is composed of tiny, indivisible particles?
In the 5th century BC, Democritus of Abdera proposed the idea of an atom, which was advanced as the smallest, indivisible particle (and the Greek word
Democritus was more a philosopher than an experimental chemist, and nothing of his writings survive. There were no practical or experimental applications of his philosophical musings. Fast forward 2000 years to the 19th century, and John Dalton proposed a simple atomic theory, which attempted to explain the behavior of matter. (If it seems that I offer disrespect to philosophy, please note that Dalton would have described himself as a natural philosopher!)
The main points of Dalton's atomic theory were: (i) that elements were composed of small indivisible particles,
A modern chemist or physicist would have very little issue with any of these principles - and of course our own conception of atoms and molecules derives from Dalton. Dalton formulated his atomic theory on experiment; he was an accomplished experimentalist. Much of his theory was based on the behaviour of gases, the which he could investigate by simple experiments.
Just in the interest of history, many speculate that Galileo and Newton in the 17th century had anticipated many of Dalton's ideas. They were such accomplished polymaths that they failed to write any of their atomic theory down.