John Dalton was a British scientist who proposed the first modern atomic theory in 1804/04%3A_Atomic_Structure/4.06%3A_Daltons_Atomic_Theory). He suggested that all matter is composed of extremely small particles called atoms, which cannot be subdivided, created, or destroyed/04%3A_Atomic_Structure/4.06%3A_Daltons_Atomic_Theory). Dalton also observed that atoms of different elements differ in size, mass, and other properties, and that chemical compounds always contain the same proportion of elements by mass, regardless of the amount/04%3A_Atomic_Structure/4.06%3A_Daltons_Atomic_Theory). Daltons atomic theory was based on the law of conservation of mass and the law of constant composition. Although Dalton did not have the necessary instruments to see or experiment on individual atoms, his theory formed the framework of modern chemistry/04%3A_Atomic_Structure/4.06%3A_Daltons_Atomic_Theory).