John Dalton was the first to create a modern atomic theory based on his experiments with atmospheric gases. Dalton’s experiments with the gases led to some of the earliest atomic masses and concepts of the atom. Such as atomic structure and reactivity.
Dalton’s theory contained three ideas: All atoms of a given element are identical. Atoms vary in different size and mass. And that atoms are indestructible.
He presented the concept that all matter was composed of different combinations of atoms. Which he calls the invisible blocks of matter.