The idea that the world around us is made up of large numbers of identical very small particles called molecules, and that the myriad of different kinds of molecules are simply differently arranged groups of atoms, is only a little over 150 years old. Yet it is in terms of this picture that any educated person views the world, and it is this picture that lies at the heart of chemistry.

The properties of both compounds and elements, especially the relative masses of the different elements that made up a compound, were studied extensively in the eighteenth century. Correspondence between chemists, formalized by publication of experimental results in the journals of scientific societies, led to the compilation of tables of composition and properties of pure compounds. It remained for the Quaker schoolmaster John Dalton to place these painstakingly collected measurements and empirical laws on the theoretical basis known as the atomic theory of matter.

John Dalton (1766-1844) first set down the understanding of the nature of matter as molecules. Dalton lived in Manchester, England, and from 1793 earned his living as a private tutor there. In the course of his studies and teaching, as well as in discussions with other members of the Literary and Philosophical Society of Manchester, he developed the ideas that led to his formulation of the atomic theory of matter in 1805.

Quantitative chemical measurements, especially those of mass, had given rise to several empirical laws that preceded the theoretical basis given to them by John Dalton. Dalton was familiar with these laws and also with the laws that had been discovered to describe the behavior of gases, as we shall see in later sections. It was to explain these empirical laws that he developed the atomic theory of matter.

The determination of the empirical mass ratios in compounds should ideally be done by decomposition to the elements (elemental analysis). However, in the eighteenth and early nineteenth centuries it was often impossible to effect the decomposition quantitatively, so many of the ratios were determined by synthesis from pure elements. Prior to the development of electrolysis, decomposition of water to hydrogen plus oxygen was not possible, but preparation or synthesis of water from hydrogen and oxygen was possible. When hydrogen was burned in excess oxygen water of constant composition was always evolved. The water could be weighed directly by absorption, the hydrogen used could be obtained by difference, and the oxygen used was established assuming conservation of mass. It can then be calculated that water is 11.19% hydrogen and 88.81% oxygen by mass. Dalton assumed that hydrogen should be assigned an atomic mass of one because it was the lightest element known. The mass of oxygen is then 88.81/11.19 = x/1, where x is the atomic mass of oxygen, and the value of 7.937 obtained were rounded to eight.

The decomposition of Fe2O3 was likewise impractical, but the reaction (rusting or oxidation) of iron in the presence of excess oxygen is simple and yields a compound of constant composition. The original iron can be weighed, as can the resulting iron oxide. The oxygen can be obtained by difference since conservation of mass is assumed. The compound has the constant composition Fe 69.9%, O 30.1% by mass. The obviously different compound FeO cannot be prepared by synthesis from the elements but must be tackled by analysis: reduction with hydrogen, yielding water and iron metal. From the mass of the original iron oxide, the mass of the water produced, and the mass of the resulting iron, the composition by mass of FeO is calculated to be 72.7% Fe and 27.3% oxygen. The atomic mass of iron is then 72.7/27.3 = x/8, or 27.9 units.