Rare Gases

Most of the rare gases have been detected in small amounts in earth minerals and in meteorites, but are found in greater abundance in the 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 the 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 one and seven percent of the naturalgas. 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.