Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought. All matter is made of atoms.
Atoms are indivisible and indestructible.
Compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms. Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory.