Elements, Chemical

Elements, Chemical, substances that cannot be decomposed, or broken into more elementary substances, by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles: electrons, protons, and neutrons.

More than 100 chemical elements are known to exist in the universe. Although several of these, the so-called transuranic elements, have not been found in nature, they have been produced artificially by bombarding the atomic nuclei of other elements with charged nuclei or nuclear particles. Such bombardment can take place in a particle accelerator such as the cyclotron, in a nuclear reactor, or in a nuclear explosion.

Chemical elements are classified as metals and non-metals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the non-metals. A group of elements called metalloids, intermediate in properties between the metals and the non-metals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the alkaline earth metals, rare earth elements, halogens, and the noble gases.

The unit for atomic weight of the elements is one-twelfth of the weight of the carbon 12 atom, which is arbitrarily set at 12. The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element.

When two atoms have the same atomic number, but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable.

The heavy transuranic elements produced in laboratories are radioactive and have very short lives. Some physicists speculate that a number of stable, superheavy elements may exist—elements with atomic numbers as high as 164, or higher, but no evidence has yet been found for such elements. The heaviest transuranic element known to have been discovered to date is element 116.