By the end of the eighteenth century, experimenters had well established that each pure substance had its own characteristic set of properties such as density, specific heat, melting point, and boiling point. Also established was the fact that certain quantitative relationships, such as the Law of Conservation of Mass, governed all chemical changes. But there was still no understanding of the nature of matter itself. Was matter continuous, like a ribbon from which varying amounts could be snipped, or was it granular, like a string of beads from which only whole units or groups of units could be removed? Some scientists believed strongly in the continuity of matter, whereas others believed equally strongly in granular matter; both reasonings were based solely on speculation.

In 1803, an English schoolmaster named John Dalton (1766-1844) summarized and extended the then-current theory of matter. The postulates of his theory, changed only slightly from their original statement, form the basis of modern atomic theory. Today, we express these four postulates as:

1. Matter is made up of tiny particles called atoms. (A typical atom has a mass of approximately 10 ^-23 g and a radius of approximately 10 ^-10 m.)

2. Over 100 different kinds of atoms are known; each kind is an element. All the atoms of a particular element are alike chemically but can vary slightly in mass and other physical properties. Atoms of different elements have different masses.

3. Atoms of different elements combine in small, whole-number ratios to form compounds. For example, hydrogen and oxygen atoms combine in a ratio of 2:1 to form the compound water, H₂O. Carbon and oxygen atoms combine in a ratio of 1:2 to form the compound carbon dioxide, CO₂. Iron and oxygen atoms combine in a ratio of 2:3 to form the familiar substance rust, Fe₂O₃.

4. The same atoms can combine in different whole-number ratios to form different compounds. As just noted, hydrogen and oxygen atoms combined in a 2:1 ratio form water; combined 1:1, they form hydrogen peroxide, H₂O₂ (Figure 3.2). Carbon and oxygen atoms combined in a 1:2 ratio form carbon dioxide; combined in a 1:1 ratio, they form carbon monoxide, CO.

FIGURE 3.2 Atoms of the same elements combine in different ratios to form different compounds.