Appendix_A.fm Page 36 Monday, June 21, 2004 6:59 AM

Elements Handbook Group 8A: Noble Gases

Physical and Chemical Properties

• All Group 8A elements are Incandescent light monatomic gases at STP. bulbs are filled with • Noble gases are colorless, argon instead of air 2 Helium | discovered in 2 odorless, and tasteless. to extend the life of He 1868 by Pierre Janssen the filament. Helium • The first compound of a 4.0026 Dominic Episcopo/ImageState noble gas, XePtF6, was 2 Neon | discovered in 1898 10 8 made in 1962. There are • In 2000, chemists in Finland Ne by Sir William Ramsay & now more than 100 known made a compound of argon, Neon Morris Travers 20.179 compounds of fluorine and HArF, that exists only at xenon. temperatures below 246ºC. 2 18 8 Argon | discovered in Ar 8 1894 by Lord Rayleigh & Argon Sir William Ramsay Melting and Boiling Points Density 39.948 0 9.73 2 10.0 36 8 Krypton | discovered in –62 18 1898 by Sir William –70 –108 Kr 8 –71 7.5 Krypton Ramsay & Morris Travers –153 –112 83.80 –140 5.90 –186 –157 5.0 3.75 2 Xenon | discovered in 54 8 –210 –189 18 1898 by Sir William –246 18 bp Density (g/L) 2.5 1.78 Xe 8 –269 Xenon Ramsay & Morris Travers (C) Temperature –249 mp 0.90 –280 –272 0.18 131.30 0 HeNe Ar Kr Xe Rn He Ne Ar Kr Xe Rn 2 Radon | discovered in 86 8 18 1900 by Friedrich E. Dorn 32 Rn 18 Radon 8 (222)

First Ionization Energy Sources Atomic Properties 2400 2372 • Helium is separated from • Noble gases have an 2080 natural gas deposits. Neon, electron configuration that 1800 2 6 2 1520 argon, krypton, and xenon ends in ns np . Helium (1s ) 1351 1170 are separated from air by is an exception. 1200 1037 fractional distillation. • Noble gases rarely form 600 • Because of its low density, compounds. When they Energy (kJ/mol) 0 helium is used in weather do, the most common HeNe Ar Kr Xe Rn balloons and airships. oxidation number is 2. • In addition to “neon” • Each noble gas emits a Noble gases have the highest ioniza- lights, noble gases are used characteristic color in a tion energies because their highest in fluorescent bulbs, strobe gas discharge tube. occupied energy levels are filled. lights, and headlights. Helium Neon Argon Krypton Xenon • Liquid helium cools the e Pictures/Getty Images magnets used for magnetic resonance imaging (MRI). enn/ Time Lif

R36 Elements Handbook Al F Appendix_A.fm Page 37 Monday, June 21, 2004 6:59 AM

Ne Neon Lights

By 1855, scientists could produce light by passing an electric current through a gas

under low pressure in a sealed glass tube. Souders/CORBIS With the discovery of the noble gases, a new aul A.

technology emerged. In 1910, George Claude P displayed the first neon lamp in Paris, France. Xe Xenon-Ion Engines In 1923, a car dealer from Los Angeles bought two signs that spelled out “Packard” for $24,000 (about $250,000 in today’s The signals for a television dollars). When he displayed the signs in Los Angeles, people program may bounce off a described the light as “liquid fire.” By the 1930s, businesses were communication satellite. The using neon lights to draw the attention of customers. satellite is in orbit above the equator. The position of the Although all noble gases emit visible light when their atoms are satellite may be maintained by excited, neon and argon are the gases most often used in neon a xenon-ion propulsion lights. Orange-red lights contain only neon. Other colors are system. produced by adding a bit of mercury to the noble gas. The tube is coated with a material that glows when exposed to UV light When electrons strike xenon emitted by mercury vapor. ■ atoms in a xenon-ion engine, the atoms lose electrons and form positive ions. The ions are accelerated by a charged Ar Taken For a Ride By Argon grid and shot from the engine at about 105 km/h. This Whether riding on a paved city street or on an unpaved moun- action pushes, or thrusts, tain trail, a bicyclist is likely to find rough patches. When the satellite in the opposite faced with rough terrain, the cyclist may worry about the tires, direction. With multiple but probably not about the bicycle frame. The frame is made engines facing in different of steel or aluminum alloy tubes that are joined together. directions, a satellite can be The tubes are joined together by Gas Tungsten Arc Welding moved in any direction. (GTAW). An electric arc is struck between a tungsten electrode Although a xenon-ion engine and the parts to be welded. Heat from the arc melts the ends of produces a relatively small the tubes and fuses them together. Filler may be placed between amount of thrust, it can the ends of the tubes to increase the strength of the joint or to provide thrust for months or produce a smoother joint. years. This makes xenon-ion During welding in air, there is a danger that the metal engines a good choice for tubes or electrode will oxidize. To prevent lengthy space missions. In oxidation, the area around the arc is addition, an inert gas poses filled with an inert gas, most often no hazard for the satellite or argon. Welding with argon has an the people who handle the added benefit. Because argon is propellant tanks. ■ a poor conductor of heat, the arc that forms is narrow. This narrow arc produces Did You Know... a weld that is both neat ? in appearance and When liquid helium is cooled to below mechanically strong. ■ 2 K, its viscosity drops to zero. It will escape from an unsealed container by flowing up the sides of the container. Tim McGuire/CORBIS Group 8A: Noble Gases R37