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helium(hē`lēəm), gaseous chemical element; symbol He; at. no. 2; at. wt. 4.0026; m.p. below −272°C; at 26 atmospheres pressure; b.p. −268.934°C; at 1 atmosphere pressure; density 0.1785 grams per liter at STPSTP
or standard temperature and pressure,
standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0°C; or 273.15°K;.
..... Click the link for more information. ; valence usually 0.
Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868. A bright yellow emission line was observed and was later shown to correspond to no known element; the new element was named by J. N. Lockyer and E. Frankland from helios [Gr.,=sun]. Helium was isolated (1895) from a sample of the uranium mineral cleveite by Sir William Ramsay.
Properties and Isotopes
Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Extremely unreactive, it is an inert gasinert gas
or noble gas,
any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. They are colorless, odorless, tasteless gases and were once believed to be entirely inert, i.e.
..... Click the link for more information. in Group 18 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. In helium obtained from natural gas about one atom in 10 million is helium-3. The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons).
Helium-4 is unusual in that it forms two different kinds of liquids. When it is cooled below 4.22°K; (its boiling point at atmospheric pressure) it condenses to liquid helium-I, which behaves as an ordinary liquid. When liquid helium-I is cooled below about 2.18°K; (at atmospheric pressure), liquid helium-II is formed. Liquid helium-II has a number of unusual properties. It is sometimes called a superfluid because it has extremely low viscosity. It also has extremely high heat conductivity and expands on cooling. It cannot be contained in an open beaker since a thin film of it creeps up the side, over the lip, and flows down the outside. The study of these phenomena is a part of low-temperature physics. When helium-3 is liquefied and cooled it does not exhibit the properties of liquid helium-II; this difference in properties between helium-3 and helium-4 can be explained in terms of quantum mechanics.
Natural Occurrence and Preparation
Helium is relatively rare and costly; it typically is produced as a by-product of the extraction of natural gas. Wells in Texas (where the Federal Helium Reserve was established in 1925 near Amarillo), Oklahoma, and Kansas are the principal world source; other sources are in Wyoming, Algeria, Qatar, and Russia. Crude helium is separated by liquefying the other gases present in the natural gas; it is then either further purified or stored for later purification and use. Some helium is extracted directly from the atmosphere; the gas is also found in certain uranium minerals and in some mineral waters, but not in economic quantities. It has been estimated that helium makes up only about 0.000001% of the combined weight of the earth's atmosphere and crust; it is most concentrated in the exosphere, which is the outermost region of the atmosphere, 600–1500 mi (960–2400 km) above the earth's surface. Helium is abundant in outer space; it makes up about 23% of the mass of the visible universe. It is the end product of energy-releasing fusion processes in starsstar,
hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.
..... Click the link for more information. (see interstellar matterinterstellar matter,
matter in a galaxy between the stars, known also as the interstellar medium. Distribution of Interstellar Matter
Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the interstellar matter
..... Click the link for more information. ).
Helium's noncombustibility and buoyancy (second only to hydrogen) make it the most suitable gas for balloons and other lighter-than-air craft. A mixture of helium and oxygen is often supplied as a breathing mixture for deep-sea divers and caisson workers and is used in decompression chambers; because helium is less soluble in human blood than nitrogen, its use reduces the risk of caisson disease, or the "bends." Helium can also be used wherever an unreactive atmosphere is needed, e.g., in electric arc welding, in growing crystals of silicon and germanium for semiconductors, and in refining titanium and zirconium metals. It is also used to pressurize the fuel tanks of liquid-fueled rockets. Liquid helium is essential for many low temperature applications (see low-temperature physicslow-temperature physics,
science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures.
..... Click the link for more information. ), and is used as a coolant in particle accelerators and MRI machines.
helium(hee -lee-ŭm) Chemical symbol: He. A chemical element with an atomic number of two. In its most abundant form it consists of a nucleus of two protons and two neutrons, around which orbit two electrons. This nucleus, i.e. the positively charged helium ion, is exceptionally stable; it is called an alpha particle. A second far less abundant isotope, helium–3, has two protons and one neutron as its nucleus. Two radioactive (i.e. unstable) isotopes also exist.
Helium is the second most abundant element in the Universe (after hydrogen): about 25% by mass and 6% or more by numbers of atoms. All but about 1% of this approximate 25% cosmic abundance is now considered to have been synthesized in the first few minutes of the Universe (see Big Bang theory). Helium is also synthesized, from hydrogen, by nuclear fusion reactions in the centers of main-sequence stars. It is by these reactions – the proton-proton chain reaction and the carbon cycle – that the energy of the stars is generated. When the hydrogen supplies in the stellar cores are exhausted, the helium is itself consumed by further fusion processes to form carbon (see stellar evolution).
Helium is not easy to detect. It was first discovered in 1868 by Norman Lockyer: a yellow emission line of a then unknown element was observed in the spectrum of the Sun's chromosphere, recorded during a solar eclipse. High temperatures are required for helium to emit or absorb radiation. Absorption lines of neutral helium (He I) do not appear, for example, in the Sun's absorption spectrum, which originates in the photosphere, but they dominate the spectra of B stars. Even greater temperatures are required to ionize helium, i.e. to remove one or both of its electrons. Singly ionized helium (He II) occurs, for instance, in O stars and in emission nebulae. Helium, although a minor component in the inner planets, is a major constituent in the atmospheres of Jupiter, Saturn, Uranus, and Neptune.
He, a chemical element in Group VIII of the periodic system. One of the inert gases. Atomic number, 2; atomic weight, 4.0026; a colorless and odorless gas. Natural helium consists of two stable isotopes: 3He and 4He (4He predominates by far).
Helium was first discovered not on earth, where there is little of it, but in the sun’s atmosphere. In 1868 the Frenchman P. J. C. Janssen and the Englishman J. N. Lockyer studied the spectroscopic composition of solar prominences. The resulting photographs contained an intense yellow line (the so-called D3-line) that could not be attributed to any of the elements known at that time. In 1871, Lockyer explained its origin by the presence in the sun of a new element, which was named helium (from the Greek helios, “Sun”). On earth, helium was isolated for the first time by the Englishman W. Ramsay from the radioactive mineral cleveite. The spectrum of the gas evolved upon heating of cleveite contained the same line.
Helium in nature. On earth there is a small amount of helium: 1 cu m of air contains only 5.24 cu cm of helium, and each kilogram of the earth’s material contains 0.003 mg of helium. However, helium is the second most common element in the universe, after hydrogen: it accounts for about 23 percent of the cosmic mass.
On earth, helium (more precisely, the isotope 4He) is being formed constantly during the disintegration of uranium, thorium, and other radioactive elements (the earth’s crust contains a total of 29 radioactive isotopes that produce helium).
About one-half of all helium is concentrated in the earth’s crust, mainly in its granite layer, in which the main reserves of radioactive elements have accumulated. The helium content of the earth’s crust is very small (3 x 10~7 percent by mass). Helium accumulates in the free gas pockets in the earth’s interior and in petroleum; such deposits may attain an industrial scale. The maximum helium concentrations (10-13 percent) have been discovered in free gaseous accumulations and in the gases of uranium deposits, as well as in gases that evolved spontaneously from underground water (20-25 percent). The greater the age of the gas-bearing rocks and the higher the content of radioactive elements in these rocks, the more helium is found in the natural gases. Volcanic gases usually contain small amounts of helium.
Helium production on an industrial scale is based on natural and petroleum gases of both hydrocarbon and nitrogenous composition. According to the raw material quality, helium deposits may be divided into the following classes: rich (He content > 0.5 percent by volume), average (0.10-0.50 percent), and poor (< 0.10 percent). In the USSR natural helium is found in many oil and gas fields. Considerable helium concentrations are known in some natural gas deposits in Canada and the USA (Kansas, Texas, New Mexico, and Utah).
The 4He isotope predominates in natural helium regardless of origin (atmospheric and from natural gas, radioactive minerals, meteorites, and other sources). The 3He content is usually low (depending on the helium source, it varies from 1.3 x 10~4 to 2 x 10~8 percent); it reaches 17-31.5 percent only in helium released from meteorites. The rate of 4He formation during radioactive decay is quite low: in 1 ton of granite containing, for example, 3 g of uranium and 15 g of thorium, 1.0 mg of helium is formed in 7.9 million years; however, since this process is continuous, it should have produced during the earth’s lifetime a much higher helium content in the atmosphere, lithosphere, and hydrosphere than that actually present (about 5 x 1014 m3). This helium deficiency is explained by its continuous escape from the atmosphere. The light helium atoms reaching the upper atmosphere layers gradually acquire speeds exceeding escape velocity and thereby become capable of overcoming the earth’s gravitational field. The simultaneous formation and escape of helium leads to a virtually unchanging concentration in the atmosphere.
The 3He isotope, in particular, is formed in the atmosphere during beta decay of the heavy hydrogen isotope tritium (T), which in turn arises from the interaction of neutrons from cosmic radiation with atmospheric nitrogen:
The 4He nuclei (consisting of two protons and two neutrons), which are called alpha particles or helions, are the most stable of the compound nuclei. The binding energy of the nucleons (protons and neutrons) in 4He has a maximum value as compared to the nuclei of other elements (28.2937 mega electron volts [MeV]); for this reason, the formation of 4He nuclei from hydrogen nuclei (protons) 1H is accompanied by the liberation of a very large amount of energy. It is believed that the nuclear reaction 41H = 4He + 2β- + 2v (the formation of 4He is accompanied by the simultaneous formation of two positrons [β+] and two neutrinos [v]) is the main source of the sun’s energy, as well as the energy of other similar stars. This process leads to the accumulation of considerable quantities of helium in the universe.
Physical and chemical properties. Under normal conditions, helium is a colorless and odorless monatomic gas. Density, 0.17846 gll; boiling point, -268.93° C. Liquid helium is the only element that does not solidify at normal pressure regardless of the temperature. The lowest pressure for the transition of liquid helium to the solid state is 2.5 meganewtons per sq m (MN/m2), or 25 atm; the melting point under these conditions is -272.1° C. The heat conduction (at 0° C)is 143.8 x 10-3 watts per cm °K [W/cm-°K)], or 343.4 x 10-6 cal/(cm.deg.sec). The atomic radius of helium as determined by various methods is 0.85 to 1.33 angstroms (Å). One liter of water at 20° C dissolves about 8.8 ml of helium. The primary ionization energy of helium—39.38 x 10~13 joules (J), or 24.58 eV—is higher than that of any other element. Helium does not have any electron affinity. Liquid helium containing only 4He exhibits a number of unique properties.
Up to the present time, attempts to prepare stable chemical compounds of helium have been unsuccessful. The existence of the He2+ ion in discharges was demonstrated spectroscopically. In 1967, the Soviet researchers V. P. Bochin, N. V. Zakurin, and V. K. Kapyshev reported the synthesis of ions of HeF+, HeF22+, and HeF2+ in the arc discharge zone by reaction of helium with fluorine, BF3, or RuF5. According to calculations, the magnitude of the energy of dissociation of the HeF+ ions is 2.2 eV.
Preparation and use. In industry, helium is produced from natural gases that contain helium (at the present time, deposits containing >0.1 percent helium are usually exploited). Helium is separated from other gases by methods of copious cooling, making use of the fact that its liquefaction is more difficult than that of any other gas.
Because of its inertness, helium is widely used for generating a protective atmosphere during smelting, cutting, and welding of reactive metals. Helium is less electrically conductive than another inert gas, argon. For this reason, an electric arc in a helium atmosphere gives higher temperatures, which significantly increases the rate of arc welding. Because of the combination of low density and incombustibility, helium is used to fill balloons. The high heat conduction of helium and its chemical inertness and very low tendency to enter into nuclear reactions with neutrons make it possible to utilize helium in cooling atomic reactors. Liquid helium is the coldest liquid on earth; it serves as a coolant in a variety of scientific studies. One of the methods for determining the absolute age of minerals is based on the measurement of the helium content of radioactive minerals. Since helium is very poorly soluble in blood, it is being used as a constituent of artificial air supplied to divers for breathing (replacement of nitrogen by helium prevents development of the bends). The use of helium in the cabin atmosphere of spaceships is under study.
S. S. BERDONOSOV and V. P. IAKUTSENI
Liquid helium. The relatively weak interaction of its atoms causes helium to remain a gas down to temperatures below the liquid range of any other gas. The maximum temperature below which it may be liquefied (its critical temperature Tc) is 5.20° K. Liquid helium is the only nonfreezing liquid: at normal pressure (see Figure 1) helium remains liquid down to any temperature, no matter how low, and freezes only at pressures exceeding 2.5 MN/m2 (25 atm).
At temperature Tλ = 2.19° K and normal pressure, liquid helium undergoes a phase transformation of the second kind. Helium above this temperature is called He I, and below it, He II. At the phase transformation temperature an anomalous increase of heat capacity (the so-called λ-point; see Figure 2), a break in the curve representing the temperature dependence of density (Figure 3), and other characteristic phenomena are observed.
In 1938, P. L. Kapitsa discovered the superfluidity of He II, which is the capability to flow virtually without viscosity. This phenomenon was explained by L. D. Landau in 1941 on the basis of quantum-mechanical concepts concerning the thermal motion in liquid helium.
At low temperatures this motion is described as the existence in liquid helium of elemental excitations, or phonons (sound quanta), which have energy Є = hv, where v is the frequency of the sound and h is Planck’s constant, and momentum p = Є/c, where c = 240 m/sec is the speed of sound. The number and energy of the phonons increase with increasing temperature T. At T > 0.6° K, excitations with high energies (rotons) appear, for which the dependence Є(p) is a nonlinear. Phonons and rotons have momentums, and hence, masses. This mass related to 1 cm3 determines the density pn of the so-called normal component of liquid helium. At low temperatures, pn approaches zero for T → 0. The motion of the normal component, as of an ordinary gas, is viscous. The remaining part of liquid helium, the so-called superfluid component, flows without friction; its density ps = p - pn. When T → Tλ, pn → p, so that at the X-point ps becomes equal to zero and the superfluidity disappears (He I is an ordinary viscous fluid).
Thus, in liquid helium, two flows with different velocities may occur simultaneously.
On the basis of these concepts it is possible to explain a number of observed facts: when He II flows out of a vessel through a narrow capillary, the temperature in the vessel increases, since it is mainly the superfluid component, which does not carry any heat with it, that flows out (the so-called mechanocaloric effect); when a temperature difference is created between the ends of a closed capillary filled with He II, motion is generated in the capillary (thermomechanical effect): the superfluid component flows from the cold end toward the hot end and is transformed there into the normal component, which moves in the opposite direction, in which case there is no net flow. Ordinary sound, as well as so-called second sound, may propagate in liquid helium. During the propagation of the second sound, a local density increase of the normal component is accompanied by a corresponding density decrease of the superfluid component.
All of the above discussion concerns the ordinary helium consisting mostly of the 4He isotope. The rarer 3He isotope has quantum properties that are different from those of 4He. Liquid 3He is also a nonfreezing liquid (Tc = 3.33° K), but it does not have superfluid properties: the viscosity of 3He increases continuously, without limit, with decreasing temperature.
L. P. PITAEVSKII
REFERENCESKeesom, W. Gelii, Moscow, 1949. (Translated from English.)
Fastovskii, V. G., A. E. Rovinskii, and Iu. V. Petrovskii. Inertnye gazy. Moscow, 1964.
Khalatnikov, I. M. Vvedenie v teoriiu sverkhtekuchesti. Moscow, 1965.
Smirnov, Iu. N. “Gelii vblizi absoliutnogo nulia.” Priroda, 1967, no. 10, p. 70.
Iakutseni, V. P. Geologiia geliia. Leningrad, 1968.