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 STP STP 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 gas inert 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 .
..... Click the link for more information. in Group 18 of the periodic table periodic 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
..... 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 rare and costly. Wells in Texas (where the Federal Helium Reserve was established in 1925 near Amarillo), Oklahoma, and Kansas are the principal world source. 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 stars star, 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 matter interstellar 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
..... Click the link for more information. ).

Uses

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 physics low-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. ).

helium

Chemical element, chemical symbol He, atomic number 2. A noble gas, it is colourless, odourless, tasteless, completely unreactive, and nontoxic. First found by spectroscopy of the Sun's atmosphere in 1868, it is the second most abundant and second-lightest element in the universe (after hydrogen). Helium makes up a tiny proportion of the atmosphere but as much as 7% of natural gas. It is the product of radioactive decay (see radioactivity) and is used in helium dating. It is used as an inert gas in welding, rocket propulsion, balloon flight, hyperbaric chambers, deep-sea diving (see nitrogen narcosis), gas chromatography, luminous signs, and cryogenics. Liquid helium, which exists only below −452 °F (−268.9 °C, about 4° C above absolute zero), is a “quantum fluid” (see fluid mechanics; quantum mechanics), with unique properties, including superfluidity, superconductivity, and near-zero viscosity.