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Each element is assigned an official symbol by the International Union of Pure and Applied Chemistry (IUPAC). For example, the symbol for carbon is C, and the symbol for silver is Ag [Lat.  argentum  = silver]. There are several ways of designating an isotope. One designation consists of the name or symbol of the element followed by a hyphen and the mass number of the isotope; thus the isotope of carbon with mass number 12 can be designated carbon-12 or C-12. The mass number is often written as a superscript, e.g., C 12 ; sometimes the atomic number is written as a subscript preceding the symbol, e.g.,  6 C 12 . The IUPAC rules for nomenclature of inorganic chemistry state that the subscript atomic number and superscript mass number should both precede the symbol, e.g.,  12 6 C. Many isotopes were given special names and symbols when they were first discovered in natural radioactive decay series (e.g., uranium-235 was called actinouranium and represented by the symbol AcU). This prac

Nonmetal, chemical element possessing certain properties by which it is distinguished from a metal . In general, this distinction is drawn on the basis that a nonmetal tends to accept electrons and form negative ions and that its oxide is acidic. Nonmetals are poor conductors of heat and electricity and do not have the luster of metals. Arsenic, antimony, selenium, and tellurium exhibit both nonmetallic and metallic properties and are called metalloids. Unlike the metals, which are all solids (with the exception of mercury) under ordinary conditions of temperature and pressure, the nonmetals appear in all three states. Argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, and xenon are normally gases. Bromine is a liquid. Boron, carbon, iodine, phosphorus, silicon, and sulfur are solids. Certain of them, e.g., boron, carbon, iodine, silicon, and sulfur, form crystals, as do the metals. In hardness they vary considerably. Carbon in its allotropic form, the dia

Properties of an element are sometimes classed as either chemical or physical. Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom's nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction. Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number. The elements are someti

Compound,  in chemistry, a substance composed of  atoms  of two or more  elements  in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often represented by its chemical formula. The formula for water is H 2 O, and for sodium chloride, NaCl. The formula weight of a compound can be determined from its formula. The molecular weight of a molecular compound can be determined from its molecular formula. Two or more distinct compounds that have the same molecular formula but different properties are called isomers. Formation and Decomposition of Compounds Compounds are formed from simpler substances by  chemical reaction . Some compounds can be formed directly from their constituent elements, e.g., water from hydrogen and oxygen: 2H 2  + O 2  → 2H 2 O. Other compounds are formed by reaction of an element with another compound; e.g., sodium hydroxide (NaOH) is formed (and hydrogen gas released) by the reaction

Synthetic elements,  in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are  technetium  (at. no. 43), which was the first element to be synthesized,  promethium  (at. no. 61),  astatine  (at. no. 85),  francium  (at. no. 87), and the  transuranium elements  (at. no. 93 and beyond in the  periodic table ). Some of these elements have since been shown to exist in minute amounts in nature, usually as short-lived members of natural radioactive decay series (see  radioactivity ). The synthetic elements through at. no. 100 ( fermium ) are created by bombarding a heavy element, such as uranium or plutonium, with neutrons or alpha particles. The synthesis of the transfermium elements (elements with at. no. 101 or greater) is accomplished by the fusion of the nuclei of two lighter elements. Elements 101 through 106 were first produced by fusing the nuclei of slightly lighter elements, such as  californium , with those

Ununhexium, artificially produced radioactive chemical element ; symbol Uuh; at. no. 116; mass number of most stable isotope 292; m.p., b.p., sp. gr., and valence unknown. Situated in Group 16 of the periodic table, it is expected to have properties similar to those of polonium and tellurium . In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create, apparently, ununoctium (element 118) atoms. The Uuo-293 isotope that they synthesized emitted an alpha particle to decay into Uuh-289, which has a life-life of about 0.6 millisecond, which then emitted an alpha particle to decay into ununquadium (element 114). Although the Berkeley laboratory retracted its claim for creating ununoctium in 2001, other research teams have since created ununhexium directly. No name has yet been adopted for element 116, which is therefore called ununhexium, from the Latin roots un for one and hex for six, under a con

Ununoctium (y'nənŏk`tēəm), artificially produced radioactive chemical element ; symbol Uuo; at. no. 118. Scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of ununoctium in experiments conducted in 2002 and 2005. They bombarded atoms of californium -249  with ions of calcium -48. Among the products of the bombardments were three atoms of ununoctium-294 (one atom in 2002 and two in 2005), each of which decayed in 0.9 milliseconds into an atom of ununhexium by emitting an alpha particle . No name has yet been adopted for element 118, which is therefore called ununoctium, from the Latin roots un for one and oct for eight, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980. In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy k