Chemical Element

Many substances now known as elements have been known since ancient times. Gold (Au) was found and made into ornaments during the late stone age, some 10,000 years ago. More than 5,000 years ago, in Egypt, the metals iron (Fe), copper (Cu), silver (Ag), tin (Sn), and lead (Pb) were also used for various purposes. Arsenic (As) was discovered around A.D. 1250, and phosphorus (P) was discovered around 1674. By 1700, about 12 elements were known, but they were not yet recognized as they are today.

The concept of elements—i.e., the theory that there are a limited number of fundamental pure substances out of which all other substances are made—goes back to the ancient Greeks. Empedocles (c. 495–435 B.C.) proposed that there are four basic "roots" of all materials: earth, air, fire, and water. Plato (c. 427–347 B.C.) referred to these four "roots" as stoicheia elements. Aristotle (384–322 B.C.), a student of Plato's, proposed that an element is "one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others." Except for nuclear fission and other nuclear reactions discovered more than 2,000 years later, by which the atoms of an element can be decomposed into smaller parts, this definition remains accurate.

Several other theories were generated throughout the years, most of which have been dispelled. For example, the Swiss physician and alchemist Theophrastus Bombastus von Hohenheim (c. 1493–1541), also known as Paracelsus, proposed that everything was made of three "principles:" salt, mercury, and sulfur. An alchemist named van Helmont (c. 1577–c.1644) tried to explain everything in terms of just two elements: air and water.

Eventually, English chemist Robert Boyle (1627–1691) revived Aristotle's definition and refined it. In 1789, French chemist Antoine Lavoisier (1743–94) was able to publish a list of chemical elements that met Boyle's definition. Even though some of Lavoisier's "ele ments" later turned out to be compounds (combinations of actual elements), his list set the stage for the adoption of standard names and symbols for the various elements.

The Swedish chemist J. J. Berzelius (1779–1848) was the first person to employ the modern method of classification: a one- or two-letter symbol for each element. These symbols could be put easily together to show how the elements combine into compounds. For example, writing two Hs and one O together as H₂O would mean that the particles (molecules) of water consist of two hydrogen atoms and one oxygen atom, bonded together. Berzelius published a table of 24 elements, including their atomic weights, most of which are close to the values used today.

By the year 1800 only about 25 true elements were known, but progress was relatively rapid throughout the nineteenth century. By the time Russian scientist Dmitri Ivanovich Mendeleev (1834–1907) organized his periodic table in 1869, he had about 60 elements to reckon with. By 1900 there were more than 80. The list quickly expanded to 92, ending at uranium (atomic number 92). There it stayed until 1940, when synthesis of the transuranium elements began.