chemical element, any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Elements are the fundamental materials of which all matter is composed.

This article considers the origin of the elements and their abundances throughout theuniverse. The geochemical distribution of these elementary substances in theEarth’s crust and interior is treated in some detail, as is their occurrence in the hydrosphere andatmosphere. The article also discusses the periodic law and the tabular arrangement of the elements based on it. For detailed information about thecompounds of the elements,seechemical compound.

General observations

At present there are 118 known chemical elements. About 20 percent of them do not exist in nature (or are present only in trace amounts) and are known only because they have been synthetically prepared in the laboratory. Of the known elements, 11 (hydrogen,nitrogen,oxygen,fluorine,chlorine, and the six noble gases) are gases under ordinary conditions, two (bromine and mercury) are liquids (two more,cesium andgallium, melt at about or just above room temperature), and the rest are solids. Elements can combine with one another to form a wide variety of more complex substances calledcompounds. The number of possible compounds is almost infinite; perhaps a million are known, and more are being discovered every day. When two or more elements combine to form acompound, they lose their separate identities, and the product has characteristics quite different from those of theconstituent elements. The gaseous elementshydrogen and oxygen, for example, with quite different properties, can combine to form the compoundwater, which has altogether different properties from either oxygen or hydrogen. Water clearly is not an element because it consists of, and actually can be decomposed chemically into, the two substances hydrogen and oxygen; these two substances, however, are elements because they cannot be decomposed into simpler substances by any known chemical process. Most samples of naturally occurring matter are physicalmixtures of compounds. Seawater, for example, is a mixture of water and a large number of other compounds, the most common of which issodium chloride, or table salt. Mixtures differ from compounds in that they can be separated into their component parts by physical processes; for example, the simple process of evaporation separates water from the other compounds inseawater.

Historical development of the concept of element

The modernconcept of an element is unambiguous, depending as it does on the use of chemical and physical processes as a means of discriminating elements from compounds and mixtures. The existence of fundamental substances from which all matter is made, however, has been the basis of much theoretical speculation since the dawn of history. The ancientGreek philosophers Thales, Anaximenes, and Heracleitus each suggested that all matter is composed of one essential principle—or element. Thales believed this element to be water; Anaximenes suggested air; and Heracleitus, fire. Another Greek philosopher,Empedocles, expressed a different belief—that all substances are composed of four elements:air, earth, fire, and water.Aristotle agreed and emphasized that these four elements are bearers of fundamental properties, dryness andheat being associated with fire, heat and moisture with air, moisture and cold with water, and cold and dryness with earth. In the thinking of these philosophers all other substances were supposed to be combinations of the four elements, and the properties of substances were thought to reflect their elementalcompositions. Thus, Greek thoughtencompassed the idea that all matter could be understood in terms of elemental qualities; in this sense, the elements themselves were thought of as nonmaterial. The Greek concept of an element, which was accepted for nearly 2,000 years, contained only one aspect of the modern definition—namely, that elements have characteristic properties.

In the latter part of the Middle Ages, asalchemists became more sophisticated in their knowledge of chemical processes, the Greek concepts of thecomposition of matter became less satisfactory. Additional elemental qualities were introduced to accommodate newly discovered chemical transformations. Thus,sulfur came to represent the quality of combustibility,mercury that of volatility or fluidity, and salt that of fixity in fire (or incombustibility). These three alchemical elements, or principles, also represented abstractions of properties reflecting the nature of matter, not physical substances.

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The important difference between a mixture and a chemical compound eventually was understood, and in 1661 the English chemistRobert Boyle recognized the fundamental nature of a chemical element. He argued that the four Greek elements could not be the real chemical elements because they cannotcombine to form other substances nor can they be extracted from other substances. Boyle stressed the physical nature of elements and related them to the compounds they formed in the modern operational way.

In 1789 the French chemistAntoine-Laurent Lavoisier published what might be considered the first list of elemental substances based on Boyle’s definition. Lavoisier’s list of elements was established on the basis of a careful, quantitative study of decomposition and recombination reactions. Because he could not devise experiments to decompose certain substances, or to form them from known elements, Lavoisier included in his list of elements such substances as lime,alumina, andsilica, which now are known to be very stable compounds. That Lavoisier still retained a measure of influence from the ancient Greek concept of the elements is indicated by his inclusion oflight and heat (caloric) among the elements.

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Seven substances recognized today as elements—gold,silver,copper,iron, lead,tin, and mercury—were known to the ancients because they occur in nature in relatively pure form. They are mentioned in the Bible and in an early Hindu medicaltreatise, theCaraka-samhita. Sixteen other elements were discovered in the second half of the 18th century, when methods of separating elements from their compounds became better understood. Eighty-two more followed after the introduction of quantitativeanalytical methods.