562 MAN IS MASTERING HIS MATERIAL WORLD The Quantitative Nature of Chemical Changes Was Demonstrated by Lavoisier. The atomic hypothesis remained a mere valueless speculation until the quantitative nature of chemical changes had been established. Bergman (1735-1784), Joseph Black (1728-1799), and Henry Cavendish (1731-1810) all made use of the balance in the study of chemical processes; but it is to the brilliant French chemist, Antoine Lavoisier (1743-1794), that is due the credit for introducing the quantitative epoch in chemistry. Until chemical changes were studied quantitatively, there was little opportunity for real progress, so Lavoisier is usually called the "father of modern chemistry." Antoine Laurent Lavoisier was born in Paris and early showed a great aptitude in scientific investigation, especially in chemistry. He made many practical contributions of importance, such as the improvement of gunpowder. Lavoisier's most important contribution was the introduction of the balance into chemical investigations. With its aid he overthrew the erroneous phlogiston theory of combustion, which assumed that whenever a substance was burned there was lost from that substance a mysterious "phlogiston." Lavoisier showed that combustion is really a combination of oxygen with a substance. In 1789 Lavoisier advanced the law of the indestructibility of matter, or what is now known as the law of conservation of mass. This law simply states that there is no perceptible gain or loss in weight in any chemical change. In 1793 Lavoisier met his death on the guillotine. Despite his great services to his country, the fact that he was a member of the aristocracy and had been associated with a hated tax-collecting company caused him to be sentenced to death by the French revolutionists. C. S. Minot said of this event: "Compared with the growth of Science, the shiftings of Governments are minor events. Until it is clearly recognized that the gravest crime of the French Revolution was not the execution of the King but the execution of Lavoisier, there is no right measure of values; for Lavoisier was one of the three or four greatest men France has produced." Other Quantitative Laws of Chemical Reaction Were Soon Discovered. The labors of many investigators in the years just following the death of Lavoisier established a second law of chemical change which was quantitative in nature, just as was Lavoisier's law of conservation of mass. This second law, called the law of definite composition, stated that the composition of a chemical compound is definite in nature; i.e., is always composed of the same elements in the same proportions by weight. it
THE ATOM IS THE UNIT OF CHEMICAL CHANGE 563 All chemical reactions were shown to be changes in which definite weights of substances react with each other to produce definite weights of products. If an excess of one of the reactants is present, it will not enter into the reaction but will reaction. The Atomic Theory Was Advanced by Dalton. remain unchanged at the end of the In 1807 John Dalton (1766-1844), an English schoolteacher, advanced his atomic theory, one of the three or four great theories of physical science. This theory may be summarized as follows: 1. The atom is the smallest division of an element which can exist within its molecules or the molecules of any of its compounds. 2. Atoms are indivisible, eternal, and indestructible. 3. Atoms of different elements differ in chemical nature and have different masses and volumes, but all atoms of the same element are alike in properties and mass. 4. Compounds are formed by combining definite whole numbers of atoms. The laws of chemical combination are readily explained by Dalton's atomic theory. Inasmuch as the atoms are assumed to be eternal and indestructible, it follows that all matter which is made up of these atoms is therefore indestructible; this is the law of conservation of mass. Inasmuch as molecules are formed by the combination of definite numbers of atoms, each of which represents a definite weight, it follows that molecules have a definite composition by weight; this is the law of definite composition. The atom is sometimes called the chemical unit of matter, while the molecule is called the physical unit. Matter rarely occurs in the form of atoms, but rather in the form of molecules, which usually consist of two or more atoms. Molecules of elements contain usually two or more atoms of the same element, while molecules of compounds necessarily contain two or more atoms of different elements. Molecules may be defined as the smallest particles of a substance that can continue to exist in a free state. Some elements, such as hydrogen, oxygen, and chlorine, may be obtained in the form of atoms, but unless something else with which these atoms may react is present, they will react with other atoms of their own kind to form molecules of these elements. A few elements — the rare gases, helium, argon, neon, krypton, xenon, and radon — which do not react, exist in the form of atoms. In this case the molecule is said to be composed of just one atom; i.e., the atom and molecule are identical. In a previous Unit the actual existence of molecules was discussed.