Elements, Chemical
In chemistry, the term element applies to any substance that can not be reduced by ordinary physical or chemical means to a simpler form. Thus, lead is an element because no method has ever been found by which the substance can be reduced into two or more parts.
The notion of elements first became popular among the natural philosophers of ancient Greece. These scholars found it difficult to imagine that nature was fundamentally as complex as it appears to be. They found it more satisfying intellectually to suppose that the wide variety of substances we encounter in nature are really composed of a small number of basic materials, that is, elements.
The Greeks devoted a great deal of thought to the question of what it is that is truly elemental in nature. Various philosophers proposed a variety of theories. In the fifth century, b.c., Empedocles (c. 490-430 b.c.) developed a view of nature based on the existence of four elements: earth, air, fire, and water. He thought that these four elements interacted with each other under the influence of two forces--love and hate--to produce all tangible materials in the universe. This concept was adopted and developed by, among others, Aristotle.
By the terms earth, air, fire, and water, Aristotle and his colleagues were not thinking of the kind of material substances that we associate with those terms. The term fire for example, represented instead a quality of heat and dryness. Water in contrast, referred to a quality of cold and moistness. Thus, to Aristotle's way of thinking, a rock might consist of all of the four basic elements, with an abundance of " earth," and lesser amounts of "air," "fire," and " water."
Aristotle's concept of an element was influential in the work of the alchemists of the Middle Ages. A basic principle in their efforts to change one material into another was to change the proportion of earth, air, fire, and water in the materials.
Scholars' ideas as to what materials are truly elemental evolved over time. For example, during the eighth century a.d. the Arabic philosopher Jabir ibn Haiyan (c. 721-815) suggested that mercury and sulfur were the only true elements.
As modern chemistry began to develop, debates over elements became more and more confused. Some scholars wanted to retain the Greek concept of a handful of fundamental elements, which might or might not be material substances. Others were aware of the increasing number of materials coming from research that seemed to be "fundamental" materials.
One of the first attempts to simplify this confusion was made by Robert Boyle in 1661. In his book, Sceptical chymist, Boyle proposed a modern definition for the term chemical element. He suggested that the term be reserved for "certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies are immediately compounded, and into which they are ultimately resolved."
Boyle's recommendation was not immediately adopted. Many chemists continued to use traditional Greek concepts to refer to elements. In fact, more than a century passed before the term chemical element was clarified to everyone's satisfaction. Then, in 1789, Antoine-Laurent Lavoisier decided to take a very pragmatic approach to the topic. In his textbook, Elements of Chemistry, he stated that he would simply call an element any substance that had not yet been reduced to simpler form by chemical means. On that basis, Lavoisier listed 33 elements. Of these, some (sulfur, phosphorus, bismuth, iron, copper, and gold, for example) were correctly identified as elements. Others (baryte, silice, chaux, and magnésie, for example) turned out to be compound substances that were only later reduced to simpler materials. And two--lumiere (light) and calorique (heat) were eventually shown to be forms of energy rather than matter.
As chemists became increasingly successful at breaking down compounds into their constituent elements, it became apparent that some elements exhibited similar properties. In addition, the properties of certain groups of elements seemed to fall into orderly series. Attempts to devise a periodic law that made some sense of these patterns went on well into the 1800s, culminating in the work of Dmitri Mendeleev and Julius Lothar Meyer.
Lavoisier's general definition of elements continues to be valid today. When a modern chemist refers to a chemical element, she has in mind a kind of substance which cannot be reduced to any simpler form by ordinary means. This is the case because each element is composed of a single kind of atom that has a different atomic number (number of protons) than the atoms of any other element.
In the two centuries following Lavoisier's definition, chemists have invested significant amounts of time and energy trying to determine precisely what substances are truly elementary. Today we know that about 90 naturally occurring substances qualify as elements. Another two dozen elements have been synthesized by scientists, but are not known to occur naturally in the universe.
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