NOBLE GAS


Meaning of NOBLE GAS in English

any of the six chemical elements that make up Group O of the periodic table. The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) (qq.v.). All of the noble gases are present in the Earth's atmosphere. With the exception of hydrogen, helium is the most plentiful element in the universe, comprising almost 25 percent of its total mass. Under ordinary conditions, the noble gases are colourless, odourless, and nonflammable. Their atoms have stable electron configurations; thus they do not easily gain or lose electrons and rarely share them with other elements. The term noble alludes to the extraordinarily limited reactivity of the gases. They also are sometimes called inert gases for the same reason. It was, however, discovered in 1962 that the heavier noble gaseskrypton, xenon, and radoncan form chemical compounds with fluorine, the strongest electron attracting of all elements. The outer electrons of the atoms of these three gases, screened from the nucleus by intervening electrons, are held less firmly and can be removed more readily from the atoms than can the electrons of helium, neon, and argon. The noble gases absorb and give off electromagnetic radiation in a much less complex manner than do other substances. This absorption and emission behaviour is exploited in the use of the gases (with the exception of highly radioactive radon) in fluorescent lighting devices and discharge lamps. If any of the noble gases is confined at low pressure in a glass tube and an electrical charge is passed through it, the gas glows. The noble gases also have very low boiling points and melting points, which make them useful as refrigerants in the study of matter at extremely low temperatures. Figure 1: Modern version of the periodic table of the elements. To see more information about an any of the six chemical elements that make up Group O of the periodic table (see Figure). The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Under ordinary conditions they are colourless, odourless, tasteless, nonflammable gases. They traditionally have been labelled Group 0 in the periodic table because for decades after their discovery it was believed that they had a valence of zero; that is, that their atoms could not combine with those of other elements to form chemical compounds. Their electronic structures and the finding that some of them do indeed form compounds suggest that a more appropriate designation would be Group VIIIa. When the members of the group were discovered and identified they were thought to be exceedingly rare, as well as chemically inactive, and therefore were called the rare gases or the inert gases. It is now known, however, that several of these elements are quite abundant on Earth and in the rest of the universe, so the designation rare is misleading. Similarly, use of the term inert has the drawback that it often is applied to gases such as nitrogen and carbon dioxide to connote their nonflammability. In chemistry and alchemy, the word noble long has signified the passivity toward oxygen of a group of metals, such as gold and platinum; it applies in the same sense to the group of gases covered here. The abundances of the noble gases decrease as their atomic numbers increase; helium is, in fact, the most plentiful element in the universe except hydrogen. All the noble gases are present in the Earth's atmosphere and, except for helium and radon, their major commercial source is the air, from which they are obtained by liquefaction and fractional distillation. Most helium is produced commercially from certain natural gas wells. Radon usually is isolated as a product of the radioactive decomposition of dissolved radium compounds (the nuclei of radium atoms spontaneously decay by emitting energy and particles; the particles are the nuclei of helium and radon atoms). Several important uses of the noble gases rest on their marked lack of chemical reactivity. Their indifference toward oxygen, for example, confers utter nonflammability upon all six noble gases. Although helium is not quite as buoyant as hydrogen, its incombustibility makes it a safer lifting gas for lighter-than-air aircraft. The noble gasesmost often helium and argon, the least expensiveare used to provide chemically unreactive environments for such operations as cutting, welding, and refining of metals (atmospheric oxygen and, in some cases, nitrogen or carbon dioxide would react with the hot metal) and in the handling of other easily attacked materials. The noble gases absorb and emit electromagnetic radiation in a much less complex way than do other substances. This behaviour is utilized in the employment of these gases in discharge lamps and fluorescent lighting devices: if any of them is confined at low pressure in a glass tube and an electrical discharge is passed through it, the gas glows. Neon produces the familiar orange-red colour of advertising signs; xenon emits a beautiful blue. The very low boiling points and melting points of the noble gases make them useful as refrigerants in the study of matter at extremely low temperatures. The low solubility of helium in fluids leads to its use in admixture with oxygen for breathing by deep-sea divers: because helium does not dissolve in the blood, it does not form bubbles upon decompression (as nitrogen does, leading to the condition known as bends). Xenon has been used as an anesthetic; although it is costly, it is nonflammable and readily eliminated from the body. Radon is highly radioactive; its only uses have been those that exploit this property, as, for example, in radiotherapy. The compounds of the noble gases are powerful oxidizing agents (substances that tend to remove electrons from others) and have potential value as reagents in the synthesis of chemical compounds. Additional reading Gerhard A. Cook (ed.), Argon, Helium, and the Rare Gases, 2 vol. (1961), contains authoritative accounts of the history, occurrence, properties, production, analytical determination, and uses of the noble gases; the work was published one year before the discovery of noble gas compounds. The question of possible hazards from environmental radon is covered in the following: M. Wilkening, Radon in the Environment (1990); Douglas G. Brookins, The Indoor Radon Problem (1990); and Leonard A. Cole, Element of Risk: The Politics of Radon (1993). The Editors of the Encyclopdia Britannica

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