The Chemical Concepts of Lavoisier

The key concepts which Lavoisier used were: the definition of a chemical element, and the qualitative description of each of the chemical elements; the qualitative description of each of the known pure chemical compounds formed by chemical combination of the different chemical elements; the systematic naming (nomenclature) of compounds on the basis of the chemical elements of which they are composed; the quantitative composition of compounds by mass, used as one of their most important distinguishing characteristics and the quantitative measurement of the heat properties of the chemical elements, of pure compounds, and of the reactions involving them Lavoisier, following Boyle, accepted the definition of an element as any substance which cannot be broken down into simpler substances. His list of elements included many of the common metallic and nonmetallic elements known today. It also included light and heat, which we would not now classify as elements. The oxides of the alkali metals and of the alkaline earths were included as elements because these oxides could not be decomposed to their respective metals prior to the later electrolytic experiments of Humphrey Davy. Lavoisier carefully described the properties of each of the elements, such as their physical form, density, and color. The description of pure compounds formed from the different elements occupied much of Lavoisier's text. Such descriptions are still included in modern chemistry textbooks. Lavoisier's descriptions were qualitative, except for the quantitative composition of the compounds by mass and the measurements of the heat properties of the compounds and the elements. The formation of oxides, and the existence of oxygen in air, was accepted by Lavoisier, and it was presented for the first time in a systematic way in his book. However, not all of Lavoisier's ideas about oxygen were correct. For example, he argued that all acids contain oxygen. While many acids do contain oxygen, some do not and as a result Lavoisier's names for compounds do not exactly match their actual structure. Nevertheless, chemists continue to use them, as we shall see in a later section. The modern systematic nomenclature of inorganic compounds is still based on that developed by Lavoisier and his colleagues in France around 1800. The quantitative composition of compounds by mass remains an important area of chemistry to this day although we now understand it on the basis of the atomic-molecular theory of matter which Lavoisier did not. As a consequence it is now presented on a theoretical rather than an empirical basis. The quantitative measurement of the heat properties which we now call enthalpy was introduced at about the time of Lavoisier's book, and some of the advances in this area are due to him. Again, this remains an important area of chemistry to this day in its own right, and formed the basis for the development of all of thermodynamics.