Helium - chemistry.

Helium - chemistry. I INTRODUCTION Helium, symbol He, colorless, odorless chemical element that occurs as a gas at room temperature. The atomic number of helium is 2, with two protons in its nucleus. The element is listed in group 18 (or VIIIa) of the periodic table (see Periodic Law). Helium is one of the noble gases and is almost chemically inert. Helium is the second most abundant element in the universe, after hydrogen. The name comes from the Greek word helios, meaning "Sun." The French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the Sun during an eclipse in 1868. Shortly afterward it was identified as an element and named by the British chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer. The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearing mineral. In 1907 the British physicist Sir Ernest Rutherford showed that alpha particles are the nuclei of helium atoms, which later investigation confirmed. II PROPERTIES OF HELIUM Helium is the lightest of all gases except hydrogen and occurs as monatomic (one atom only) molecules. Helium becomes a liquid at -268.9°C (-451.8°F), in a phase called helium I; below -270.98°C (-455.476°F), it changes to a phase called helium II and becomes a superfluid. Helium solidifies at -272.2°C (-457.9°F) at pressures above 26 atmospheres; helium boils at -268.9°C (-452.0°F) and has a density of 0.1664 g/liter at 20°C (68°F) and one atmosphere. The atomic weight of helium is 4.0026. The stable isotopes (nuclei with different mass numbers) of helium are helium-3 (2 protons + 1 neutron), a rare form, and helium-4 (2 protons + 2 neutrons), the most common form. The isotopes helium-5 through helium-10 are unstable and rapidly decay. These heavy isotopes are only known from certain nuclear reactions and from experiments with particle accelerators. Helium, like the other noble gases, is almost chemically inert. Its single electron shell is filled, making possible reactions with other elements extremely difficult and the resulting compounds quite unstable. However, under extreme pressure some chemical reactions may occur. Molecules of compounds with neon, another noble gas, and with hydrogen have been detected, however, and other compounds have been suggested, such as a combination of hydrogen-helium-iron. Because of helium's abundance in the universe, the existence of such reactions, however rare, could be of importance in cosmology. Helium is the most difficult of all gases to liquefy and is impossible to solidify at atmospheric pressure. These properties make liquid helium extremely useful as a refrigerant and for experimental work in producing and measuring temperatures close to absolute zero. Liquid helium can be cooled almost to absolute zero at normal pressure by rapid removal of the vapor above the liquid. At a temperature slightly above absolute zero, it is transformed into a phase called helium II, also called superfluid helium, a liquid with unique physical properties thought to result from quantum mechanical effects. It has no freezing point, and its viscosity is apparently zero; it passes readily through minute cracks and pores and will even creep up the sides and over the lip of a container. Helium-3, the lighter helium isotope of mass 3, which has an even lower boiling point than ordinary helium, exhibits markedly different properties when liquefied. See Superfluidity. When helium is cooled to near absolute zero under extreme pressure, it can behave like a solid. Some experiments indicate that at extremely low temperatures solid helium has unusual properties that suggest a possible new state of matter called supersolidity. Scientists are investigating the existence of supersolidity. Solid helium becomes stiffer in a supersolid state but also shows features of possible "perpetual flow" similar to superfluidity. III OCCURRENCE OF HELIUM Helium makes up about 9 percent of the atoms in the universe, compared to hydrogen at about 90 percent of atoms. Helium also accounts for about 25 percent of the mass of the universe, compared to about 73 percent from hydrogen. Heavier elements make up the remaining amounts. Most of the helium in the universe was created about 13.7 billion years ago shortly after the big bang, the event that began the universe (see Nucleosynthesis). This primordial helium exists as gas in interstellar space and in stars. Primordial helium also made up some of the gas that formed our solar system about 4.6 billion years ago. This ancient gas is still found in the Sun and in atmospheres of gas-giant planets such as Jupiter and Saturn, and is thought to exist in parts of the mantles of rocky planets such as Earth and Venus. Volcanic eruptions may still release small amounts of primordial helium left in Earth's mantle. Helium nuclei are also created in the cores of stars through the fusion of hydrogen nuclei and other reactions. However, much of this stellar helium is processed into heavier elements through nucleosynthesis. About 9 percent of cosmic rays are helium nuclei called alpha particles. Helium nuclei (alpha particles) are also released in certain types of radioactive decay--nearly all the helium released on Earth's surface is the result of the decay of radioactive elements in Earth's crust and mantle, with a small amount derived from primordial helium in the mantle. Helium is rare on Earth, primarily found mixed with natural gas trapped in underground pockets. Once helium is released it is so light it escapes Earth's atmosphere into space and cannot be recovered. At sea level, helium occurs in the atmosphere at 5.4 parts per million. The proportion increases slightly at higher altitudes. About 1 part per million of atmospheric helium consists of helium-3, now thought to be a product of the decay of tritium, a radioactive hydrogen isotope of mass 3. The common helium isotope, helium-4, probably comes from radioactive alpha emitters in rocks. Natural gas, which contains an average of 0.4 percent helium, is the major commercial source of helium. By far the largest users of helium are agencies of the United States government. IV USES OF HELIUM Because it is noncombustible, helium is preferred to hydrogen as the lifting gas in lighter-than-air balloons and airships; it has 92 percent of the lifting power of hydrogen, although it weighs twice as much. Helium is used to pressurize and stiffen the structure of rockets before takeoff and to pressurize the tanks of liquid hydrogen or other fuel in order to force fuel into the rocket engines. It is useful for this application because it remains a gas even at the low temperature of liquid hydrogen. A potential use of helium is as a heat-transfer medium in nuclear reactors because it remains chemically inert and nonradioactive under the conditions that exist within the reactors. Liquid helium is important as a coolant to reach extremely low temperatures needed for cryogenic research and to study superconductivity, the state when electric currents flow with virtually no resistance. Elements that can become superconductors using liquid helium include mercury, arsenic, and niobium. The liquid form of helium also serves as a coolant for superconductor magnets used in particle accelerators that study subatomic particles. In astronomy, liquid helium is the coolant for space telescopes and satellites that detect electromagnetic energy in the infrared and microwave wavelengths. Helium is used in place of nitrogen as part of the synthetic atmosphere breathed by deep-sea divers, caisson workers, and others, because it reduces susceptibility to the bends. This synthetic atmosphere is also used in medicine to relieve sufferers of respiratory difficulties because helium moves more easily than nitrogen through constricted respiratory passages. In surgery, beams of ionized helium from synchrocyclotron sources are proving useful in treating eye tumors, by stabilizing or even shrinking the tumors. Such beams are also used to shrink blood-vessel malformations in the brains of patients. Helium is used in inert-gas arc welding for light metals, such as aluminum and magnesium alloys, that might otherwise oxidize; the helium protects heated parts from attack by air. High-tech applications for helium include nuclear magnetic resonance, mass spectroscopy, fiber optics, and computer microchip production. Helium is transported as a gas in small quantities, compressed in heavy steel cylinders. Larger amounts of helium can be shipped as a liquid in insulated containers, thus saving shipping costs. Helium is a nonrenewable resource and cannot be created artificially in any practical way. The gas is important to many types of scientific research and may be essential to future advanced technologies. Helium was not stockpiled by the U.S. government between 1973 and 1980, nor were private natural-gas producers required to recover helium from their wells. It is estimated that some 370 million cu m (about 13 billion cu ft) of helium were lost each year during that time. Scientists have called for more efforts to conserve and recycle helium as the supply decreases and prices increase. The United States is the world's largest supplier of helium but its reserves may be depleted in the near future. The largest remaining helium reserves are thought to be in Russia, which may become the world's main supplier in coming decades. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.