noble gas


noble gas
any of the chemically inert gaseous elements of group 8A or 0 of the periodic table: helium, neon, argon, krypton, xenon, and radon. Also called inert gas.
[1900-05]

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Any of the six chemical elements that make up the rightmost group of the periodic table as usually arranged: helium, neon, argon, krypton, xenon, and radon.

All are colourless, odourless, and nonflammable and occur in tiny amounts in the atmosphere (though helium is the most plentiful element in the universe). Their stable electronic configurations, with no unpaired electrons to share, make them extremely unreactive
hence "noble" (i.e., aloof) or inert
though the three heaviest, with outer electrons held less firmly, can form compounds (mainly with fluorine). These gases absorb and give off electromagnetic radiation in a much less complex way than other substances, a property exploited in their use in fluorescent lighting devices and discharge lamps. They glow with a characteristic colour when confined in a transparent container at low pressure with an electric current passing through it. Their very low boiling and melting points make them useful as refrigerants for low-temperature research (see cryogenics).

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Introduction
 any of the seven chemical elements (chemical element) that make up Group 18 (VIIIa) of the periodic table (periodic law). The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and element 118 (temporarily named ununoctium [Uuo]). The noble gases are colourless, odourless, tasteless, nonflammable gases (gas). They traditionally have been labeled Group 0 in the periodic table because for decades after their discovery it was believed that they could not bond to other atoms (atom); that is, that their atoms could not combine with those of other elements to form chemical compounds (chemical compound). Their electronic structures and the finding that some of them do indeed form compounds has led to the more appropriate designation, Group 18.

      When the members of the group were discovered and identified, they were thought to be exceedingly rare, as well as chemically inert, and therefore were called the rare or 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 connotes chemical passivity, suggesting that compounds of Group 18 cannot be formed. In chemistry and alchemy, the word noble has long signified the reluctance of metals (metal), such as gold and platinum, to undergo chemical reaction; it applies in the same sense to the group of gases covered here.

      The abundances of the noble gases decrease as their atomic numbers (atomic number) increase. Helium is the most plentiful element in the universe except hydrogen. All the noble gases are present in 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 radium compounds. The nuclei (nucleus) of radium atoms spontaneously decay by emitting energy and particles, helium nuclei (alpha particles (alpha particle)) and radon atoms.

History
 In 1785 Henry Cavendish (Cavendish, Henry), an English chemist and physicist, found that air contains a small proportion (slightly less than 1 percent) of a substance that is chemically less active than nitrogen. A century later Lord Rayleigh (Rayleigh, John William Strutt, 3rd Baron), an English physicist, isolated from the air a gas that he thought was pure nitrogen, but he found that it was denser than nitrogen that had been prepared by liberating it from its compounds. He reasoned that his aerial nitrogen must contain a small amount of a denser gas. In 1894, Sir William Ramsay (Ramsay, Sir William), a Scottish chemist, collaborated with Rayleigh in isolating this gas, which proved to be a new element— argon.

      After the discovery of argon, and at the instigation of other scientists, in 1895 Ramsay investigated the gas released upon heating the mineral clevite, which was thought to be a source of argon. Instead, the gas was helium, which in 1868 had been detected spectroscopically in the Sun but had not been found on Earth. Ramsay and his coworkers searched for related gases and by fractional distillation of liquid air discovered krypton, neon, and xenon, all in 1898. radon was first identified in 1900 by German chemist Friedrich E. Dorn; it was established as a member of the noble-gas group in 1904. Rayleigh and Ramsay won Nobel Prizes (Nobel Prize) in 1904 for their work.

      In 1895 the French chemist Henri Moissan (Moissan, Henri), who discovered elemental fluorine in 1886 and was awarded a Nobel Prize in 1906 for that discovery, failed in an attempt to bring about a reaction between fluorine and argon. This result was significant because fluorine is the most reactive element in the periodic table. In fact, all late 19th- and early 20th-century efforts to prepare chemical compounds (chemical compound) of argon failed. The lack of chemical reactivity implied by these failures was of significance in the development of theories of atomic structure. In 1913 the Danish physicist Niels Bohr (Bohr, Niels) proposed that the electrons (electron) in atoms (atom) are arranged (electronic configuration) in successive shells having characteristic energies and capacities and that the capacities of the shells for electrons determine the numbers of elements in the rows of the periodic table. On the basis of experimental evidence relating chemical properties to electron distributions, it was suggested that in the atoms of the noble gases heavier than helium, the electrons are arranged in these shells in such a way that the outermost shell always contains eight electrons, no matter how many others (in the case of radon, 78 others) are arranged within the inner shells.

 In a theory of chemical bonding advanced by American chemist Gilbert N. Lewis (Lewis, Gilbert N.) and German chemist Walther Kossel in 1916, this octet of electrons was taken to be the most stable arrangement for the outermost shell of any atom. Although only the noble-gas atoms possessed this arrangement, it was the condition toward which the atoms of all other elements tended in their chemical bonding. Certain elements satisfied this tendency by either gaining or losing electrons outright, thereby becoming ions (ion); other elements shared electrons, forming stable combinations linked together by covalent bonds (covalent bond). The proportions in which atoms of elements combined to form ionic or covalent compounds (their “valences (valence)”) were thus controlled by the behaviour of their outermost electrons, which—for this reason—were called valence electrons (valence electron). This theory explained the chemical bonding of the reactive elements, as well as the noble gases' relative inactivity, which came to be regarded as their chief chemical characteristic. (See also chemical bonding: Bonds between atoms (chemical bonding).)

      Screened from the nucleus by intervening electrons, the outer (valence) electrons of the atoms of the heavier noble gases are held less firmly and can be removed (ionized) more easily from the atoms than can the electrons of the lighter noble gases. The energy required for the removal of one electron is called the first ionization energy. In 1962, while working at the University of British Columbia, British chemist Neil Bartlett discovered that platinum hexafluoride would remove an electron from (oxidize) molecular oxygen to form the salt [O2+][PtF6]. The first ionization energy of xenon is very close to that of oxygen; thus Bartlett thought that a salt of xenon might be formed similarly. In the same year, Bartlett established that it is indeed possible to remove electrons from xenon by chemical means. He showed that the interaction of PtF6 vapour in the presence of xenon gas at room temperature produced a yellow-orange solid compound then formulated as [Xe+][PtF6]. (This compound is now known to be a mixture of [XeF+][PtF6], [XeF+] [Pt2F11], and PtF5.) Shortly after the initial report of this discovery, two other teams of chemists independently prepared and subsequently reported fluorides of xenon—namely, XeF2 and XeF4. These achievements were soon followed by the preparation of other xenon compounds and of the fluorides of radon (1962) and krypton (1963).

      In 2006, scientists at the Joint Institute for Nuclear Research in Dubna, Russia, announced that element 118, the next noble gas, had been made in 2002 and 2005 in a cyclotron. (Most elements with atomic numbers greater than 92—i.e., the transuranium elements (transuranium element)—have to be made in particle accelerators.) No physical or chemical properties of element 118 can be directly determined since only three atoms of element 118 have been produced.

General properties of the group
      Each noble-gas element is situated in the periodic table between an element of the most electronegative group, the halogen elements (halogen element) (Group 17, the atoms (atom) of which add electrons (electron) to achieve the octet and thereby become negative ions (ion)), and an element of the most electropositive group, the alkali metals (alkali metal) (Group 1, the atoms of which lose electrons to become positive ions).

 Several important uses of the noble gases (gas) depend on their reluctance to react chemically. Their indifference toward oxygen, for example, confers utter nonflammability upon the noble gases. Although helium is not quite as buoyant as hydrogen, its incombustibility makes it a safer lifting gas for lighter-than-air craft. The noble gases—most often helium and argon, the least expensive—are used to provide chemically unreactive environments for such operations as cutting, welding, and refining of metals (metal) such as aluminum (atmospheric oxygen and, in some cases, nitrogen or carbon dioxide would react with the hot metal).

 The noble gases absorb and emit electromagnetic radiation in a much less complex way than do other substances. This behaviour is used in discharge lamps and fluorescent lighting devices: if any noble gas is confined at low pressure in a glass tube and an electrical discharge is passed through it, the gas will glow. neon produces the familiar orange-red colour of advertising signs; xenon emits a beautiful blue colour.

      Noble gases have uses that are derived from their other chemical properties. The very low boiling points (boiling point) and melting points (melting point) of the noble gases make them useful in the study of matter at extremely low temperatures (temperature). The low solubility of helium in fluids leads to its admixture with oxygen for breathing by deep-sea divers (underwater diving): because helium does not dissolve in the blood, it does not form bubbles upon decompression (as nitrogen does, leading to the condition known as decompression sickness, or the bends (decompression sickness)). Xenon has been used as an anesthetic; although it is costly, it is nonflammable and readily eliminated from the body. radon is highly radioactive (radioactivity); its only uses have been those that exploit this property (e.g., radiation therapy). (Element 118 is also radioactive, but, since only three atoms (atom) of this element have thus far been observed, its physical and chemical properties cannot be documented.)

      Only krypton, xenon, and radon are known to form stable compounds. The compounds of these noble gases are powerful oxidizing agents (substances that tend to remove electrons (electron) from others) and have potential value as reagents in the synthesis of other chemical compounds (chemical compound).

Gary J. Schrobilgen

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 historical background relating to the discovery of noble-gas reactivity and the impact of this discovery on modern chemistry are dealt with in P. Laszlo and G.J. Schrobilgen, “One or Several Pioneers? The Discovery of Noble-Gas Compounds,” Angewandte Chemie, International Edition in English, 27:479–489 (1988). An overview of the complex noble-gas species that are formed by the removal (cation) or addition (anion) of fluoride to a neutral noble-gas fluoride or oxide fluoride is presented in Henry Selig and J.H. Holloway, “Cationic and Anionic Complexes of the Noble Gases,” Topics in Current Chemistry, 124:33–90 (1984). Boris Žemva, “Noble Gases,” in Encyclopedia of Inorganic Chemistry, 5:2660–80 (1994); and G.J. Schrobilgen “Noble Gas Chemistry” in Encyclopedia of Physical Science and Technology, 10:449–461, 3rd ed. (2002), summarize the chemistry of the noble-gas elements.Gary J. Schrobilgen

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Universalium. 2010.

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