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Noble Gases
Noble gases
Photo by: photo-dave
The noble gases are the six elements that make up Group 18 of the periodic       table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and       radon (Rn). At one time, this family of elements was also known as the       rare gases. Their present name comes from the fact that the six gases are       highly unreactive; they appear almost "noble"—above       interacting with other members of the periodic table. This lack of       reactivity has also led to a second name by which they are sometimes       known—the inert gases. (Inert means inactive.)
Abundance and production
As their former name suggests, the noble gases are rather uncommon on       Earth. Collectively, they make up about 1 percent of Earth's        atmosphere. Most of the noble gases have been detected in small amounts       in minerals found in Earth's crust and in meteorites. They are       thought to have been released into the atmosphere long ago as by-products       of the decay of radioactive elements in Earth's crust.       (Radioactivity is the property that some elements have of spontaneously       giving off energy in the form of particles or waves when their nuclei       disintegrate.)
Of all the rare gases, argon is present in the greatest amount. It makes       up about 0.9 percent by volume of Earth's atmosphere. The other       noble gases are present in such small amounts that it is usually more       convenient to express their concentrations in terms of parts per million       (ppm). The concentrations of neon, helium, krypton, and xenon are,       respectively, 18 ppm, 5 ppm, 1 ppm, and 0.09 ppm. For example, there are       only 5 liters of helium in every million liters of air. By contrast,       helium is much more abundant in the Sun, stars, and outer space. In fact,       next to hydrogen, helium is the most abundant element in the universe.       About 23 percent of all atoms found in the universe are helium atoms.
Radon is present in the atmosphere in only trace amounts. However, higher       levels of radon have been measured in homes around the United States.       Radon can be released from soils containing high concentrations of       uranium, and they can be trapped in homes that have been weather sealed to       make heating and cooling systems more efficient. Radon testing
Lead canisters used to store xenon for medical diagnostic purposes. (Reproduced by permission of
Photo Researchers, Inc.
)
kits are commercially available for testing the radon content of household       air.
Most of the rare gases are obtained commercially from liquid air. As the       temperature of liquid air is raised, the rare gases boil off from the       mixture at specific temperatures and can be separated and purified.       Although present in air, helium is obtained commercially from natural gas       wells where it occurs in concentrations of between 1 and 7 percent of the       natural gas. Most of the world's helium supplies come from wells       located in Texas, Oklahoma, and Kansas. Radon is isolated as a product of       the radioactive decay of radium compounds.
Properties
The noble gases are all colorless, odorless, and tasteless. They exist as       monatomic gases, which means that their molecules consist of a single atom       apiece. The boiling points of the noble gases increase in moving down the       periodic table. Helium has the lowest boiling point of any element. It       boils at 4.215 K (−268.93°C). It has no melting point       because it cannot be frozen at any temperature.
The most important chemical property of the noble gases is their lack of       reactivity. Helium, neon, and argon do not combine with any other elements       to form compounds. It has been only in the last few decades that compounds       of the other rare gases have been prepared. In 1962 English chemist Neil       Bartlett (1932–) succeeded in preparing the first compound of a       noble gas, a compound of xenon. The compound was xenon platinofluoride       (XePtF 6 ). Since then, many xenon compounds containing mostly fluorine or oxygen       atoms have also been prepared. Krypton and radon have also been combined       with fluorine to form simple compounds. Because some noble gas compounds       have powerful oxidizing properties, they have been used to synthesize       other compounds.
The low reactivity of the noble gases can be explained by their electronic       structure. The atoms of all six gases have outer energy levels containing       eight electrons. Chemists believe that such arrangements are the most       stable arrangements an atom can have. Because of these very stable       arrangements, noble gas atoms have little or no tendency to gain or lose       electrons, as they would have to do to take part in a chemical reaction.
Uses
As with all substances, the uses to which the noble gases are put reflect       their physical and chemical properties. For example, helium's low       density and inertness make it ideal for use in lighter-than-air craft such        as balloons and dirigibles (zeppelins). Because of the element's       very low boiling point, it has many applications in low-temperature       research and technology. Divers breathe an artificial oxygen-helium       mixture to prevent the formation of gas bubbles in the blood as they swim       to the surface from great depths. Other uses for helium have been in       supersonic wind tunnels, as a protective gas in growing silicon and       germanium crystals and, together with neon, in the manufacture of gas       lasers.
Neon is well known for its use in neon signs. Glass tubes of any shape can       be filled with neon. When an electrical charge is passed through the tube,       an orange-red glow is emitted. By contrast, ordinary incandescent       lightbulbs are filled with argon. Because argon is so inert, it does not       react with the hot metal filament and prolongs the bulb's life.       Argon is also used to provide an inert atmosphere in welding and       high-temperature metallurgical processes. By surrounding hot metals with       inert argon, the metals are protected from potential oxidation by oxygen       in the air.
Krypton and xenon also find commercial lighting applications. Krypton can       be used in incandescent lightbulbs and in fluorescent lamps. Both are also       employed in flashing stroboscopic lights that outline commercial airport       runways. And because they emit a brilliant white light when electrified,       they are used in photographic flash equipment. Due to the radioactive       nature of radon, it has medical applications in radiotherapy.