John Dalton (1766–1844), an English natural philosopher, chemist, physicist, and teacher, developed the concepts of an atomic theory in the early nineteenth century. He published A New System of Chemical Philosophy in 1808. His main concepts of atomic theory may be summarized as:

• All matter—solid, liquid and gas—consists of tiny, indivisible particles called atoms.
• All atoms of a given element have the same mass and are identical, but are different from the atoms of different elements.
• Chemical reactions involve the rearrangement of combinations of those atoms, not the destruction of atoms.
• When elements react to form compounds, they combine in simple, whole-number ratios.