| Helium - He
CAS: 7440-59-7 Description: Light, odorless, colorless, tasteless gas Classification:Noble Gas Date of Discovery: 1895 Discoverer: Sir William Ramsey Name Origin: Greek hêlios, "the sun" |
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Atomic Number: 2
Number of Neutrons: 2 Atomic Mass: 4.002602(2) amu Melting Point: -272.2 °C (26 atm) Boiling Point: -268.93 °C Density (0 °C, 1 atm): 0.1785 g/cm3 7.62 lb/ft3 (liquid, at b.p.) Atomic volume: Electrical resistivity: Thermal conductivity: 0.00152 W/cmK Enthalpy of atomization: Enthalpy of vaporization: 0.0845 kJ/mol Enthalpy of fusion: Specific heat capacity: 5.193 J/gK |
Energy levels: 2
Electron configuration: 1s2 Crystal Structure: Hexagonal Atomic radius: 0.49 Å Covalent radius: 0.93 Å Oxidation States: usually 0 Electronegativity, Pauling: Electron affinity: not stable First ionization energy: 24.587 eV 2nd ionization energy: 54.416 eV Polarizability: 0.204956 10-24cm3 |
| Isotope | Natural Abundance | Atomic Mass | Half-life | Decay Mode | Spin |
| 3He | 1.37 x 10-4 | 3.01602931 | Stable | 1/2+ | |
| 4He |  100. |
4.00260325 | Stable | 0+ | |
| 5He | 5.01222 | 7.6 x 10-22 s | n,  - |
3/2- | |
| 6He | 6.018888 | 0.807 s |  - |
0+ | |
| 7He | 7.02803 | 3 x 10-21 s | n | (3/2)- | |
| 8He | 8.03392 | 0.119 s | n, - |
0+ | |
| 9He | 9.0438 | n | |||
| 10He | 2n | ||||
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Evidence of the existence of helium was first obtained by Janssen during the solar eclipse of 1868 when he detected a new line in the solar spectrum; Lockyer and Frankland suggested the name helium for the new element; in 1895, Ramsay discovered helium in the uranium mineral cleveite (a variety of uraninite UO2), and it was independently discovered in cleveite by the Swedish chemists Cleve and Langlet about the same time. Rutherford and Royds in 1907 demonstrated that alpha particles are helium nuclei. Except for hydrogen, helium is the most abundant element found throughout the universe. Helium is extracted from natural gas; all natural gas contains at least trace quantities of helium. It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. The fusion of hydrogen into helium provides the energy of the hydrogen bomb. The helium content of the atmosphere is about 1 part in 200,000. While it is present in various radioactive minerals as a decay product, the bulk of the Free World's supply is obtained from wells in Texas, Oklahoma, and Kansas. The only known helium extraction plants, outside the United States, in 1996 were in Eastem Europe (Poland), Russia, China, and India. The cost of helium fell from $2500/ft3 in 1915
to 1.5¢/ft3 in 1940. Commercial helium in 1996 was priced at about $70/300 ft3. (8.5 m3). Helium has the lowest melting point of any element and has found wide use in cryogenic research, as its boiling point is close to absolute zero. Its use in
the study of superconductivity is vital. Using liquid helium, Kurti and co-workers, and others, have succeeded in obtaining temperatures of a few microkelvins by the adiabatic
demagnetization of copper nuclei, starting from about 0.01 K. Seven isotopes of helium are known. Liquid helium (He4) exists in two forms: He4I and He4II, with a sharp transition point at 2.174 K (3.83 cm Hg). He4I (above this temperature) is a normal liquid, but He4II (below it) is unlike any other known substance. It expands on cooling; its conductivity for heat is enormous; and neither its heat conduction nor viscosity obeys normal rules. It has other peculiar properties. Helium is the only liquid that cannot be solidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but it can readily be solidified by increasing the pressure. Solid 3He and 4He are unusual in that both can readily be changed in volume by more than 30% by application of pressure. The specific heat of helium gas is unusually high. The density of helium vapor at the normal boiling
point is also very high, with the vapor expanding greatly when heated to room temperature. Containers filled with helium gas at 5 to 10 K should be treated as though they contained liquid helium due to the large increase in pressure resulting from warming the gas to room temperature. While helium normally has a 0 valence, it seems to have a weak tendency to combine with certain other elements. Means of preparing helium diflouride have been studied, and species such as HeNe and the molecular ions He+ and He++ have been investigated. Helium is widely used as an inert gas shield for arc welding; as a protective gas in growing silicon and germanium crystals, and in titanium and zirconium
production; as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. A mixture of helium and oxygen is used as an artificial atmosphere for divers and others working under pressure. Different ratios of He/02 are used for different depths at which the diver is operating. Helium is extensively used for filling balloons as it is a much safer gas than hydrogen. One of
the recent largest uses for helium has been for pressuring liquid fuel rockets. A Saturn booster such as used on the Apollo lunar missions required about 13 million ft3 of helium for a firing, plus more for checkouts. Liquid helium's use in magnetic resonance imaging (MRI) continues to increase as the medical profession
accepts and develops new uses for the equipment. This equipment is providing accurate diagnoses of problems where exploratory surgery has previously been required to determine problems. Another medical application that is being developed uses MRI to determine by blood analysis whether a patient has any form of cancer. Lifting gas applications are increasing. Various companies in addition to Goodyear, are now using "blimps" for advertising. The Navy and the Air Force are investigating the use of airships to provide early warning systems to detect low-flying cruise missiles. The Drug Enforcement Agency has used radar-equipped blimps to detect drug smugglers along the southern border of the U.S. In addition, NASA is currently using helium-filled balloons to sample the atmosphere in Antarctica to determine what is depleting the ozone layer that protects Earth from harmful U.V. radiation. Research on and development of materials which become superconductive at temperatures well above the boiling point of heliumcould have a major impact onthe demand for helium. Less costly refrigerants having boiling points considerably higher could replace the present need to cool such superconductive materials to the boiling point of helium.
LINKS: Helium Isotope Laboratory How a Helium Balloon Works Return
Sources for the information on this website include: Lide, David R., ed. CRC Handbook of Chemistry and Physics, 78th Ed., 1997-1998. |
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