The rare gases, also known as the noble gases or the inert gases, are a group of six gaseous elements found in small amounts in the atmosphere: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Collectively they make up about one percent of Earth’s atmosphere. They were discovered by scientists near the end of the nineteenth century, and because they were so unreactive were initially called the inert gases. Although rare gases is used often to describe these elements, they are only rare in abundance relative to other gases found in the atmosphere of Earth.
Helium was the first of the rare gases to be discovered. In fact, its discovery is unique among the elements since it is the only element to be first identified in another part of the solar system before being discovered on the Earth. In 1868, Pierre Janssen (1824–1907), a French astronomer, was observing a total solar eclipse from India. Janssen used an instrument called a spectroscope to analyze the sunlight. The spectroscope broke the sunlight into lines which were characteristic of the elements emitting the light. He saw a previously unobserved line in the solar spectrum, which indicated the presence of a new element that Janssen named helium after the Greek word helios, meaning sun. A quarter of a century later, Scottish chemist and physicist William Ramsay (1852–1916) studied gases emitted from radioactive uranium ores. With help from two British experts on spectroscopy, William Crooks (1832–1919) and Norman Lockyer (1836–1920), the presence of helium in Earth-bound minerals was confirmed. Shortly thereafter, helium was also detected as a minor component in Earth’s atmosphere.
The discovery of the remaining rare gases is credited to two men, Ramsay and Lord Rayleigh (1842– 1919). Beginning in 1893, Rayleigh observed discrepancies in the density of nitrogen obtained from different sources. Nitrogen obtained from the air (after removal of oxygen, carbon dioxide, and water vapor) always had a slightly higher density than when prepared from a chemical reaction (such as heating certain nitrogen-containing compounds). Ramsay eventually concluded that the nitrogen obtained from chemical reactions was pure, but nitrogen extracted from the air contained small amounts of an unknown gas which accounted for the density discrepancy. Eventually it was realized that there were several new gases in the air. The method used to isolate these new gaseous elements involved liquefying air (by subjecting it to high pressure and low temperature) and allowing the various gases to boil off at different temperatures. The names given to the new elements were derived from Greek words that reflected the difficultly in isolating them: Ne, neos (new); Ar, argos (inactive); Kr, kryptos (hidden); Xe, xenon (stranger). Radon, which is radioactive, was first detected as a gas released from radium and, subsequently, identified in air. Ramsay and Rayleigh received Nobel Prizes in 1904 for their scientific contributions in discovering and characterizing the rare gases.
The rare gases form group 18 of the periodic table of elements. This is the vertical column of elements on the extreme right of the periodic table. As with other groups of elements, the placement of all the rare gases in the same group reflects their similar properties. The rare gases are all colorless, odorless, and tasteless. They are also monatomic gases which means that they exist as individual atoms.
The most noticeable feature of the rare gases is their lack of chemical reactivity. Helium, neon, and argon do not combine with any other atoms to form compounds, and it has been only in the last few decades that compounds of the other rare gases have been prepared. In 1962, English-born American chemist Neil Bartlett (1932–), then at the University of British Columbia, succeeded in the historic preparation of the first compound of xenon. 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 rare gas compounds have powerful oxidizing properties (they can remove electrons from other substances) they have been used to synthesize other compounds.
The low reactivity of the rare gases is due to the arrangement of electrons in the rare gas atoms. The configuration of electrons in these elements makes them very stable and therefore unreactive. The reactivity of any element is due, in part, to how easily it gains or loses electrons, which is necessary for an atom to react with other atoms. The rare gases do not readily do either. Prior to Bartlett’s preparation of the first xenon compound, the rare gases were widely referred to as the inert gases. Because the rare gases are so unreactive, they are harmless to living organisms. Radon, however, is hazardous because it is radioactive.
Most of the rare gases have been detected in small amounts in Earth minerals and in meteorites, but are found in greater abundance in Earth’s atmosphere. 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. Of all the rare gases, argon is present in the greatest amount, about 0.9% by volume. This means there are 0.2 gal (0.9 l) of argon in every 26.4 gal (100 l) of air. By contrast, there are 20 gal (78 l) of nitrogen and 5.5 gal (21 l) of oxygen gas in every 26.4 gal (100 l) of air. The other rare 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, 5, 1, and 0.09 ppm. For example, there are only 1.32 gal (1.5 l) of helium in every million liters of air. By contrast, helium is much more abundant in the Sun and stars and consequently, next to hydrogen, is the most abundant element in the universe. 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 can be trapped in homes that have been weather sealed to make heating and cooling systems more efficient. Radon testing kits are commercially available for testing the radon content of household air.
Most of the rare gases are commercially obtained 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 commercially obtained from natural gas wells where it occurs in concentrations of between 1 to 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.
The properties of each rare gas dictate its specific commercial applications. Because they are the most abundant, and therefore the least expensive to produce, helium and argon find the most commercial applications. Helium’s low density and inertness make it ideal for use in lighter-than-air craft, such as balloons and blimps. Although helium has nearly twice the density of hydrogen, it has about 98% of hydrogen’s lifting power. A little over 324.7 gal (1,230 l) of helium lifts 2.2 lb (1 kg). Helium is also nonflammable and therefore considerably safer than hydrogen, which was once widely used in gas-filled aircraft. Liquid helium has the lowest boiling point of any known substance (about -452°F; -269°C) and therefore has many low-temperature applications in research and industry. Divers breathe an artificial oxygen-helium mixture to prevent gas bubbles forming 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, to make gas lasers.
Neon is well known for its use in neon signs. Glass tubes of any shape can be filled with neon and when an electrical charge is passed through the tube, an orangered glow is emitted. By contrast, ordinary incandescent light bulbs 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 light bulbs and
Density— The amount of mass of a substance per unit volume. A less dense substance floats in a more dense substance; helium will rise in air.
Oxidation— A type of chemical reaction occurring whenever electrons are removed from a substance.
Periodic table— A chart listing all the known elements. It is arranged so that elements with similar properties fall into one of eighteen groups. The rare gases are found in group 18. In older versions of the periodic table, this group is numbered 0, or VIII A.
Spectroscope— A device that breaks light from hot atoms into a spectrum of individual wavelengths. Each element has its own spectrum and can therefore be identified with this instrument.
in fluorescent lamps. Both are also employed in flashing stroboscopic lights that outline commercial airport runways. Because they emit a brilliant white light when electrified, they are also used in photographic flash equipment. Due to the radioactive nature of radon, it has found medical applications in radiotherapy.
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Nicholas C. Thomas