Helium is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most abundant element in the universe, significant amounts are found on Earth only in natural gas. It is used in cryogenics, in deep-sea breathing systems, for inflating balloons, and as a protective gas for many purposes. Helium is not toxic and has no biological effect.

hydrogen – helium   He Ne       Full table
General
Name, Symbol, Number	Helium, He, 2
Atomic weight	4.002602
Chemical series	Noble gases
Group, Period, Block	18 (VIIIA), 1, p
Density (0°C, 1 atm)	0.179 g/L
Appearance	colorless
Thermal data
Melting point (at 26 atm)	0.95 K (-272.2°C)
Boiling point	4.22 K (-268.93°C)
Specific heat capacity	5193 J/(kg*K)]]
Thermal conductivity	0.152 W/(m*K)]]
Heat of vaporization	0.0845 kJ/mol
Heat of fusion	5230 J/mol
Electronic data
Electron configuration	1s2
Electrons per shell	2
Valence	0
1st ionization potential	2372.3 kJ/mol
2nd ionization potential	5250.5 kJ/mol
Steric data
Covalent radius	32 pm
van der Waals radius	140 pm
Molar volume	21.0 ×10-6 m3/mol
Crystal structure	hexagonal
Isotopes
iso abundance half-life DM DE MeV DP 3He 0.000137% Stable with 1 neutron 4He 99.999863% Stable with 2 neutrons 6He synthetic 806.7 ms ²- 3.508 6Li
All conditions STP except where noted.

History

Helium was first detected in 1868 as a bright yellow line in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a solar eclipse in India. The same year, English astronomer Norman Lockyer also observed a previously unknown yellow line in the solar spectrum and concluded that it was caused by an element unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun, helios. In 1895, British chemist William Ramsay isolated helium on Earth by treating cleveite with mineral acids. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by Swedish chemists Per Teodor Cleve and Nils Langlet.

In 1905, American chemists Hamilton Cady and David McFarland discovered that helium could be extracted from natural gas. In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is a helium nucleus. Helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes in 1908 by cooling the gas to less than one kelvin. It was first solidified in 1926 by his student Willem Hendrik Keesom. In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. In 1972, the same phenomenon was observed in helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.

Helium II

Helium II exhibits characteristics of two distinct fluids, one a normal, viscous liquid and the other a superfluid apparently without internal friction. It has a mobile, rapid flow through even the smallest of capillaries and, in the fountain effect, can rise over the rim of a containment vessel in a thin film that appears unaffected by gravity. In addition, its thermal conductivity is greater than that of any other known substance. When introduced, heat will rapidly propagate through the substance in waves, a phenomenon called second sound.

Reactions

Helium is chemically unreactive under all normal conditions. With electric glow discharge or electron bombardment, however, helium can form compounds with tungsten, iodine, fluorine, sulfur and phosphorus.

Isotopes

Although there are six known isotopes of helium, only helium-3 and helium-4 are stable. The others, radioactive, rapidly decay into other substances. The most common isotope, helium-4, is produced by alpha decay from heavier radioactive elements; its nucleus is an alpha particle. It is an unusually stable nucleus because its nucleons are arranged into complete shells. There is only a trace amount of helium-3 on Earth, produced from the beta decay of tritium.

Abundance

Helium is the second most abundant element in the known universe after hydrogen and constitutes nearly a quarter of the mass of the universe. It is concentrated in the stars, where it is formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. According to the Big Bang model of the early development of the universe, the vast majority of helium was formed in the first three minutes after the Big Bang.

However, the concentration of helium in the Earth's atmosphere is only 1 part in 200,000, largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. All considerable helium on Earth is a result of radioactive decay. The decay product is found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl. There are also small amounts in mineral springs, volcanic gas, meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived. The principal source in the world is the natural gas wells of the American states of Texas, Oklahoma, and Kansas.

Synthesis

Helium can be synthesized by bombardment of lithium or boron with high-velocity protons.

Uses

Pressurized helium is commercially available, extracted from natural gas.

Helium is used for many purposes:

• Because it is lighter than air, airships and balloons are inflated with helium for lift. Helium is advantageous in airships because it is not flammable and has 92.64% of the lifting power of the alternative hydrogen.
• The voice of a person who has inhaled helium becomes temporarily high-pitched, because the speed of sound in helium is nearly thrice that in air, with a corresponding increase in the resonant frequencies of the larynx. Although this is a novel amusement, it can be lethally dangerous with concentrated helium.
• Trimix, an atmosphere of helium, oxygen, and nitrogen, is used in deep-sea breathing systems to reduce the risk of nitrogen narcosis and oxygen toxicity at high pressures.
• Because of its extremely low melting and boiling points, helium is used as a coolant in magnetic resonance imaging, nuclear reactors, superconducting magnets, and cryogenics.
• Because it is inert, helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in arc welding, and in gas chromatography.
• Helium used to pressurize liquid fuel rockets.
• Helium is used in supersonic wind tunnels.
• The gain medium of the helium-neon laser is a mixture of helium and neon.

External links

• WebElements: Helium (http://www.webelements.com/webelements/elements/text/He/key.html)
• EnvironmentalChemistry.com – Helium (http://environmentalchemistry.com/yogi/periodic/He.html)
• It's Elemental – Helium (http://education.jlab.org/itselemental/ele002.html)
• Hoax: Helium Causes Death (CIAC Hoaxbusters) (http://hoaxbusters.ciac.org/HBUrbanMyths.shtml#helium)