The Atomic Structure of Matter

The most important of all chemical theories is the atomic theory. In 1805 the English chemist and physicist John Dalton (1766–1844), of Manchester, stated the hypothesis that substances consist of small particles of matter. He called these particles atoms, from the Greek word «atomos», meaning indivisible. This hypothesis gave a simple explanation or picture of previously observed but unsatisfactorily explained relations among the weights of substances taking part in chemical reactions with one another. It was necessary that the hypothesis be confirmed. Hadn't it been verified by further work in chemistry and physics it wouldn't have become the atomic theory. The existence of atoms is now accepted as a fact.

All ordinary matter consists of atoms. The exceptional kinds of matter are the elementary particles from which atoms are made (electrons, protons, neutrons) and similar sub-atomic particles (positrons, mesons). But atoms are the units which retain their identity when chemical reactions take place, and therefore they are important to us now. Atoms are the structural units of all solids, liquids, and gases. They are very small ‑ only about 2 Å to 5 Å in diameter.

This is indeed small. If a piece of rock, or anything else, one inch in diameter were magnified to the size of the earth, its constituent atoms would become about the size of golf balls or tennis balls.

Every atom consists of one nucleus and one or more electrons. The nucleus is a small, heavy particle containing almost all the mass of the atom. Nuclei are very small indeed. The nucleus of an atom is only about one ten-thousandth as great in diameter as the atom itself, and the volume of the nucleus is one million-millionth, of the volume of the atom.

If nuclei could be packed together side by side, they would give a form of matter with very great density. The electron is a particle with a small mass, 1/1845 that of the lightest nucleus, and with a negative electrical charge.

The electron itself is about as large as a nucleus, its diameter being about 10-12 cm. The electrons in an atom are attracted by the nucleus. The electrons in an atom move rapidly around in the space extending over a diameter of a few Å about the nucleus, and because they move about so fast they effectively fill this space in such a way as to repel any other atom which approaches to within this diameter.

Were it not for the rapid progress of scientific knowledge about atoms the evidence for the existence of atoms would not be so overwhelming.