Dalton's atomic theory was the first attempt to describe all matter in terms of atoms and their properties. He based his theory on the law of conservation of mass and the law of constant composition. The first part of theory states that all matter is made of atoms, which are indivisible. The second part says that all atoms of a given element are identical in mass and properties. The third part says compounds are combinations of two or more different types of atoms. The fourth and final part of theory states that a chemical reaction is a rearrangement of atoms. Through the course of the theory, parts had to be modified based on new discovery of subatomic particles and isotopes.