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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 (see also CHEMISTRY; ELECTRON; NEUTRON; PROTON). The total number of known chemical elements is 115. The naturally occurring elements are those with atomic numbers from 1 (hydrogen) to 92 (uranium). Elements with atomic numbers higher than 92 are called Transuranium Elements. Except for traces of neptunium (at.no. 93) and plutonium (at.no. 94), the rest of all the known transuranium elements (23 as of 1999) have been produced artificially by bombarding the atomic nuclei of other elements with charged nuclei or nuclear particles. Such bombardment can take place in an accelerator such as the cyclotron, in a nuclear reactor, or in a nuclear explosion (see Particle Accelerators). Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, 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-the proton-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-the atomic mass unit (amu)-is one-twelfth of the weight of the carbon-12 atom, the standard by which atomic weights or masses of other elements are computed (see Atom And Atomic Theory).
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 (see Isotope; Radioactivity). All transuranium elements are radioactive and have very short half-lives. Although physicists theorized as early as the 1960s that a number of superheavy elements with a higher degree of stability may exist, such elements were not produced until the 1990s. S.Z.L. For further information on this topic, see the Bibliography, sections General chemistry, Chemical elements. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
