Dalton's Atomic Theory. John Dalton was an English scientist who is well known for his work in the development of atomic theory. All matter, whether an element, a compound or a mixture are made up of very tiny particles called atoms. All atoms of a given element are identical in size, mass and chemical properties. Atoms are indivisible particles which can neither be created nor destroyed in a chemical reaction. In a chemical reaction, atoms only get rearranged. An atom of an element does not change into the atom of another element.