History of the Atomic Theory

From ChemPRIME

The atomic, microscopic way of looking at matter is actually a fairly new development. The United States has already celebrated its two-hundredth birthday, whereas the atomic theory is only about 175 years old. None of the Founding Fathers—not even Benjamin Franklin, the most scientific of them—thought about matter in terms of atoms.

Neither, for that matter, did the man whose experiments and ideas led directly to the theory itself. Antoine Lavoisier[1] was born in 1743, the same year as Thomas Jefferson. The son of a wealthy French lawyer, he was well educated and became a successful businessman, gentleman farmer, economist, and social reformer, as well as the leading chemist of his day. It was Lavoisier’s position as a tax collector, not his chemical research, which led to his death by guillotine in 1794, at the height of the French Revolution. Much of Lavoisier’s work as a chemist was devoted to the study of combustion. He became convinced that when a substance is burned in air, it combines with some component of the air. Eventually he realized that this component was the dephlogisticated air which had been discovered by Joseph Priestly (1733 to 1804) a few years earlier. Lavoisier renamed this substance oxygen. In an important series of experiments he showed that when mercury is heated in oxygen at a moderate temperature, a red substance, calx of mercury, is obtained. (A calx is the ash left when a substance burns in air.) At a higher temperature this calx decomposes into mercury and oxygen. Lavoisier’s careful experiments also revealed that the combined masses of mercury and oxygen were exactly equal to the mass of calx of mercury. That is, there was no change in mass upon formation or decomposition of the calx. Lavoisier hypothesized that this should be true of all chemical changes, and further experiments showed that he was right. This principle is now called the law of conservation of mass.

As Lavoisier continued his experiments with oxygen, he noticed something else. Although oxygen combined with many other substances, it never behaved as though it were itself a combination of other substances. Lavoisier was able to decompose the red calx into mercury and oxygen, but he could find no way to break down oxygen into two or more new substances. Because of this he suggested that oxygen must be an element—an ultimately simple substance which could not be decomposed by chemical changes.

Lavoisier did not originate the idea that certain substances (elements) were fundamental and all others could be derived from them. This had first been proposed in Greece during the fifth century B.C. by Empedocles, who speculated that all matter consisted of combinations of earth, air, fire, and water. These ideas were further developed and taught by Aristotle and remained influential for 2000 years.

Lavoisier did, however, produce the first table of the elements which contained a large number of substances that modern chemists would agree should be classifies as elements. The accompanying figure displays a list taken from the first English edition of Lavoisier’s “Textbook of Chemistry” (1790). He published it with the knowledge that further research might succeed decomposing some of the substances listed, thus showing them not to be elements. One of his objectives was to prod his contemporaries into just that kind of research. Sure enough the “earth substances” listed at the bottom were eventually shown to be combinations of certain metals with oxygen. It is also interesting to note that not even Lavoisier could entirely escape from Aristotle’s influence. The second element in his list is Aristotle’s “fire,” which Lavoisier called “caloric,” and which we now call “heat.” Both heat and light, the first two items in the table, are now regarded as forms of energy rather than of matter.

Although his table of elements was incomplete, and even incorrect in some instances, Lavoisier’s work represented a major step forward. By classifying certain substances as elements, he stimulated much additional chemical research and brought order and structure to the subject where none had existed before. His contemporaries accepted his ideas very readily, and he became known as the father of chemistry. Vitz 15:09, 3 September 2009 (UTC)