Elements and Atoms: Elements

What is an element? Perusal of the reference shelf of a local library is likely to turn up two quite different definitions. Recent editions of such general-purpose works as Encyclopedia Britannica (15th ed., 1989) and the Oxford English Dictionary (2nd ed. CD-ROM, 1992) define a chemical element as a substance that cannot be decomposed into simpler substances by ordinary chemical processes. The McGraw-Hill Dictionary of Scientific and Technical Terms (5th ed., 1994), however, defines an element as a substance made up of atoms with the same atomic number. The Concise Encyclopedia Chemistry(de Gruyter, 1994) gives a similar definition, adding, "This definition has supplanted the classical definition of an E. as a substance which cannot be decomposed into simpler substances by chemical or physical means." The difference in definitions, then, represents a change over time. (The Oxford English Dictionary definition conveys a strong sense of anachronism by its parenthetical observation that more than 70 elements are known, a statement which has been true for over a century. Yet the newer definition has not completely displaced the classical one even from introductory chemistry textbooks [Roundy 1988].)

What should we make of these two quite different-sounding definitions? After all, the list of elements discovered and characterized by scientists using the older definition are still elements under the newer definition. Even so, the different definitions embody important conceptual differences. The classical definition evokes macroscopic processes of analytical "wet" chemistry, and is silent on the structure of elements; the modern definition is a microscopic one, inseparable from an understanding of the structure of matter and of particular details of the atomic nucleus. The alteration in definition reminds us that as knowledge changes, so does the language which expresses that knowledge. Sometimes new words are used to express new ideas; sometimes old words are given new meanings.

The selections presented in the first part of this book follow the term element still deeper into the past than the "classic" definition mentioned above. The book begins with a pre-scientific notion, the four elements of the ancients. Aristotle's explanation of the elements does not exemplify scientific reasoning. Aristotle did employ empirical observations in some of his writings, but did not make them the foundation of his philosophical system. Reading Aristotle can be quite difficult because the concepts behind words such as element and matter have changed so much in the intervening time. The next selection jumps ahead two millennia to Robert Boyle, who insisted on experiments and observations as the basis for deciding what were elements. Boyle's definition of element is almost the "classical" definition given above. A century after Boyle, Antoine Lavoisier formulated that classical definition, in a selection which also includes interesting observations on the role of language in conveying scientific knowledge. The final three selections in this section illustrate the downfall from elementary status of each of the four elements of the ancients. Joseph Priestley was not the first to distinguish among different kinds of "airs" (gases); however, his work along those lines is voluminous. His description of the gas we now call oxygen is reproduced here. Lavoisier's work on combustion, which relied greatly on the discovery of oxygen, initiated the understanding of fire as a process and not a material, let alone an element. His explanation of the related process of calcination showed that metals were simpler substances than their "earths" or metal oxides. Finally, the compound nature of water was demonstrated by several investigators in the 1780s. The section concludes with a selection describing Lavoisier's work on the subject.

Reference

Willard H. Roundy, Jr., "What is an element?" Journal of Chemical Education 66, 729-30 (1989)

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