1. Atom and Atomic Theory

Atomic Theory is the study of the nature of atoms and the forces which hold them together. In ancient Greek philosophy the word atom was used to describe the smallest bit of matter that could be conceived. This "fundamental particle," to use the present-day term for this concept, was thought of as indestructible; in fact, the Greek word for atom means "not divisible." Knowledge about the size and nature of the atom grew

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slowly throughout the centuries when people were content merely to speculate about it.

With the advent of experimental science in the 16th and 17th centu­ries progress in atomic theory quickened. Chemists soon recognized that all liquids, gases, and solids could be analyzed into their ultimate com­ponents, or elements. For example, salt was found to be composed of two distinct and different elements, sodium and chlorine, which are joined together in an intimate form known as a chemical compound. Air was discovered to consist of a mixture of the gases nitrogen and oxygen. Water was symbolized as HOH, meaning that it consists of two atoms of hydrogen for every atom of oxygen.

2. Dalton's Theory

John Dalton, a British schoolmaster and chemist, was fascinated by the patchwork puzzle of the elements. Early in the 19th century he made studies of the way in which the various elements combine with one an­other to form chemical compounds. Other scientists, among them the English physicist Sir Isaac Newton, had already speculated that the smallest units of a substance are atoms. Dalton was regarded as the founder of atomic theory because he made the theory quantitative. He showed how these atoms link together in definite proportions. Subse­quent investigations proved that the smallest unit of a chemical substance such as water is a molecule. Each molecule of water consists of a single atom of oxygen and two atoms of hydrogen joined by an electrical force called a "chemical bond."

All atoms of any given element behave in the same way chemically. Thus, from a chemical viewpoint, the atom is the smallest entity to be considered. The chemical properties of the various elements are quite different; their atoms combine in many different ways to form a multi­tude of different chemical compounds. Some elements, such as the gases helium and argon, are inert, that is, they fail to react with other elements. Unlike oxygen, which has a diatomic molecule (two atoms combined in a single molecule), helium and other inert gases are monatomic elements, with a single atom per molecule.

3. Avogadro's Law

The study of gases attracted the attention of the Italian physicist Amedeo Avogadro, who in 1811 formulated an important law bearing his name. This law states that equal volumes, of different gases contain the same number of molecules when compared under the same condi-

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tions of temperature and pressure. Given these conditions, two identical bottles, one filled with oxygen and the other with helium, will contain exactly the same number of molecules. Twice as many atoms of oxygen will be present, however, because oxygen is diatomic.