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The noble gases are the elements in group 18 (also sometimes Group 0 IUPAC Style, or Group 8) of the periodic table. The group is also called the helium family or neon family. Chemically, the noble gases are very stable due to having the maximum number of valence electrons their outer shell can hold. Noble gases rarely react with other elements since they are already stable. Under normal conditions, they occur as odourless, colourless, monatomic gases. Each of them has its melting and boiling point close together, so that only a small temperature range exists for each noble gas in which it is a liquid. Noble gases have numerous important applications in lighting, welding and space technology.

The seven noble gases are: helium, neon, argon, krypton, xenon, radon, and ununoctium.


“Noble gas” is the translation of the German Edelgas, which was in use as early as 1898. This refers to the extremely low level of reactivity under normal conditions. The noble gases have also been referred to as inert gases, but these terms are not strictly accurate because several of them do take part in chemical reactions. Another old term is rare gases, although argon forms a fairly considerable part (0.93% by volume, 1.29% by mass) of the Earth’s atmosphere.


The existence of noble gases was not known until after the advent of the periodic table. In the late nineteenth century, Lord Rayleigh discovered that some samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with scientist William Ramsay, Lord Rayleigh theorized that the nitrogen extracted from air was associated with another gas, argon. With this discovery, they realized that a whole class of gases was missing from the periodic table. Eventually, all the known noble gases except for helium were discovered in the air, with argon being much more common than the others, and the table was completed. Helium was detected spectrographically in the Sun in 1868. The isolation of helium on Earth had to wait until 1895. Under standard conditions, the noble gases all occur as monatomic gases.

Chemical make-up

Noble gases have full valence electron shells. Valence electrons are the outermost electrons of an atom and are normally the only electrons which can participate in chemical bonding. According to atomic theory derived from quantum mechanics and experimental trends, atoms with full valence electron shells are extraordinarily stable and therefore do not form chemical bonds.

All of them exhibit an extremely low chemical reactivity and very few noble gas compounds have been prepared. No conventional compounds of helium or neon have yet been prepared, while xenon and krypton are known to show some reactivity in the laboratory. Recently argon compounds have also been successfully characterised. The noble gases’ lack of reactivity can be explained in terms of them having a “complete valence shell”. They have little tendency to gain or lose electrons. The noble gases have high ionization energies and negligible electro negativities The noble gases have very weak inter-atomic forces of attraction, and consequently very low melting points and boiling points. This is why they are all monatomic gases under normal conditions, even those with larger atomic masses than many normally solid elements.


One of the most commonly encountered uses of the noble gases in everyday life is in lighting. Argon is often used as a suitable safe and inert atmosphere for the inside of filament light bulbs, and is also used as an inert atmosphere in the synthesis of air and moisture sensitive compounds (as an alternative for nitrogen). Some of the noble gases glow distinctive colours when used inside lighting tubes ( neon lights). Helium, due to its non reactivity (compared with flammable hydrogen) and lightness, is often used in blimps and balloons. Helium and argon are commonly used to shield a welding arc, and the surrounding base metal from the atmosphere during welding. Krypton is also used in lasers, which are used by doctors for eye surgery. Xenon is used in xenon arc lamps, and it has anaesthetic properties.

Noble gas notation

The noble gases can be used in conjunction with the electron configuration notation to make what is called the Noble Gas Notation. For example: while the electron notation of the element carbon is 1s²2s² 2p², the Noble Gas notation would be [He] 2s²2p².

This notation makes the identification of elements faster, and is shorter and easier than writing out the full notation of orbitals.

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HELIUM 1 of 3   Leave a comment

Helium (He) is a colourless, odourless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. In its most common form, helium-4, it has two neutrons in its nucleus, while a second, rarer, stable isotope called helium-3 contains just one neutron. The behaviour of liquid helium-4’s two fluid phases, helium I and helium II, is important to researchers studying quantum mechanics (in particular the phenomenon of super fluidity) and to those looking at the effects that temperatures near absolute zero have on matter (such as superconductivity).

In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. It is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). A much less serious use is to temporarily change the timbre and quality of one’s voice by inhaling a small volume of the gas (see precautions section below).

Helium is the second most abundant and second lightest element in the known universe, and is one of the elements believed to have been created in the Big Bang. In the modern universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth helium is rare, and almost all of that which exists was created by the radioactive decay of much heavier elements ( alpha particles are helium nuclei). After its creation, part of it was trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by fractional distillation. Large reserves of helium have been found in the natural gas fields of the United States (the largest supplier) but helium is known in gas reserves of a few other countries.


Notable characteristics


Gas and plasma phases


Helium is the least reactive member of the noble gas elements, and thus also the least reactive of all elements; it is inert and monatomic in virtually all conditions. Due to helium’s relatively low molar (molecular) mass, in the gas phase it has a thermal conductivity, specific heat, and sound conduction velocity that are all greater than any gas, except hydrogen. For similar reasons, and also due to the small size of its molecules, helium’s diffusion rate through solids is three times that of air and around 65% that of hydrogen.


Helium is less water soluble than any other gas known, and helium’s index of refraction is closer to unity than that of any other gas. Helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 40 K at 1 atmosphere) does it cool upon free expansion. Once pre cooled below this temperature, helium can be liquefied through expansion cooling.


Throughout the universe, helium is found mostly in a plasma state whose properties are quite different from atomic helium. In a plasma, helium’s electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, they interact with the Earth’s magnetosphere giving rise to Birkeland currents and the aurora.


Solid and liquid phases


Helium solidifies only under great pressure. The resulting colourless, almost invisible solid is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%. With a bulk modulus on the order of 5×107 Pa it is 50 times more compressible than water. Unlike any other element, helium will fail to solidify and remain a liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure. It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure.


Solid helium has a density of 0.214 ±0.006 g/ml (1.15 K, 66 atm) with a mean isothermal compressibility of the solid at 1.15 K between the solidus and 66 atm of 0.0031 ±0.0008/atm. Also, no difference in density was noted between 1.8 K and 1.5 K. This data projects that T=0 solid helium under 25 bar of pressure (the minimum required to freeze helium) has a density of 0.187 ±0.009 g/ml.


Helium I state


Below its boiling point of 4.22 kelvin and above the lambda point of 2.1768 kelvin, the isotope helium-4 exists in a normal colourless liquid state, called helium I. Like other cryogenic liquids, helium I boils when it is heated. It also contracts when its temperature is lowered until it reaches the lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again.


Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is. This colourless liquid has a very low viscosity and a density one-eighth that of water, which is only one-fourth the value expected from classical physics. Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This is probably due to its boiling point being so close to absolute zero, which prevents random molecular motion (heat) from masking the atomic properties.


Helium II state


Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. The isotope helium-3 also has a super fluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3.


Helium II is a super fluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10−7 to 10−8 m width it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are super fluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.


Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin. As a result of this creeping behaviour and helium II’s ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the Van der Waals force. These waves are known as third sound.


In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which super fluid helium leaks easily but through which non superfluid helium cannot pass. If the interior of the container is heated, the super fluid helium changes to non superfluid helium. In order to maintain the equilibrium fraction of super fluid helium, super fluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.


The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called second sound.


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Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small containers called dewars which hold up to 1,000 litres of helium, or in large ISO containers which have nominal capacities as large as 11,000 gallons (41,637 litres). In gaseous form, small quantities of helium are supplied in high pressure cylinders holding up to 300 standard cubic feet, while large quantities of high pressure gas are supplied in tube trailers which have capacities of up to 180,000 standard cubic feet.


Because it is lighter than air, airships and balloons are inflated with helium for lift. In airships, helium is preferred over hydrogen because it is not flammable and has 92.64% of the buoyancy (or lifting power) of the alternative hydrogen.

For its low solubility in water, the major part of human blood, mixtures of helium with oxygen and nitrogen ( trimix), with oxygen only ( heliox), with common air ( heliair), and with hydrogen and oxygen ( hydreliox), are used in deep-sea breathing systems to reduce the high-pressure risk of nitrogen narcosis.


At extremely low temperatures, liquid helium is used to cool certain metals to produce superconductivity, such as in superconducting magnets used in magnetic resonance imaging. Helium at low temperatures is also used in cryogenics.

For its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a coolant in some nuclear reactors, such as pebble-bed reactors.

Helium is used as a shielding gas in arc welding processes on materials that are contaminated easily by air. It is especially useful in overhead welding, because it is lighter than air and thus floats, whereas other shielding gases sink.

Because it is inert, helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in gas chromatography, and as an atmosphere for protecting historical documents. This property also makes it useful in supersonic wind tunnels.


In rocketry, helium is used as an ullage medium to displace fuel and oxidisers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 13 million cubic feet (370,000 m³) of helium to launch.


The gain medium of the helium-neon laser is a mixture of helium and neon.

Because it diffuses through solids at a rate three times that of air, helium is used as a tracer gas to detect leaks in high-vacuum equipment and high-pressure containers, as well as in other applications with less stringent requirements such as heat exchangers, valves, gas panels, etc.


Because of its extremely low index of refraction, the use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes.


The age of rocks and minerals that contain uranium and thorium, radioactive elements that emit helium nuclei called alpha particles, can be discovered by measuring the level of helium with a process known as helium dating.


The high thermal conductivity and sound velocity of helium is also desirable in thermo acoustic refrigeration. The inertness of helium adds to the environmental advantage of this technology over conventional refrigeration systems which may contribute to ozone depleting and global warming effects.

Because helium alone is less dense than atmospheric air, it will change the timbre (not pitch) of a person’s voice when inhaled. However, inhaling it from a typical commercial source, such as that used to fill balloons, can be dangerous due to the risk of asphyxiation from lack of oxygen, and the number of contaminants that may be present. These could include trace amounts of other gases, in addition to aerosolized lubricating oil.




Scientific discoveries

Evidence of helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometres in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 line, for it was near the known D1 and D2 lines of sodium, and concluded that it was caused by an element in the Sun unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun, ????? (helios).


On 26 March 1895 British chemist William Ramsay isolated helium on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulphuric acid, noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay’s discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.


In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei, by allowing them to penetrate the thin glass wall of a evacuated tube, then creating a discharge in the tube to study the spectra of the new gas inside. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed, because helium does not have a triple point temperature where the solid, liquid, and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keesom by subjecting helium to 25 atmospheres of pressure.


In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 (a boson) has almost no viscosity at temperatures near absolute zero, a phenomenon now called super fluidity This phenomenon is related to Bose-Einstein condensation. In 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to Cooper pairs of electrons producing superconductivity.


Extraction and uses

After an oil drilling operation in 1903 in Dexter, Kansas, U.S. produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas contained, by volume, 72% nitrogen, 15% methane—insufficient to make the gas combustible, 1% hydrogen, and 12% of an unidentifiable gas. With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium. Far from being a rare element, helium was present in vast quantities under the American Great Plains, available for extraction from natural gas.


This put the United States in an excellent position to become the world’s leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable lifting gas. A total of 200,000 cubic feet (5700 m³) of 92% helium was produced in the program even though only a few cubic feet (less than 100 litres) of the gas had previously been obtained. Some of this gas was used in the world’s first helium-filled airship, the U.S. Navy’s C-7, which flew its maiden voyage from Hampton Roads, Virginia to Bolling Field in Washington, D.C. on 1 December 1921.


Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. Helium was also vital in the atomic bomb Manhattan Project.


The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas with the goal of supplying military airships in time of war and commercial airships in peacetime. Due to a US military embargo against Germany that restricted helium supplies, the Hindenburg was forced to use hydrogen as the lift gas. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.


After the “Helium Acts Amendments of 1960” (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a 425-mile (684 km) pipeline from Bushton, Kansas to connect those plants with the government’s partially depleted Cliffside gas field, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified.


By 1995, a billion cubic metres of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve. The resulting “Helium Privatization Act of 1996” (Public Law 104–273) directed the United States Department of the Interior to start liquidating the reserve by 2005.


Helium produced before 1945 was about 98% pure (2% nitrogen), which was adequate for airships. In 1945 a small amount of 99.9% helium was produced for welding use. By 1949 commercial quantities of Grade A 99.995% helium were available.


For many years the United States produced over 90% of commercially usable helium in the world. Extraction plants created in Canada, Poland, Russia, and other nations produced the remaining helium. In the mid 1990s, A new plant in Arzew, Algeria producing 600mmcf came on stream, with enough production to cover all of Europe’s demand. Subsequently, in 2004–2006 two additional plants, one in Ras Laffen, Qatar and the other in Skikda, Algeria were built, but as of early 2007, Ras Laffen is functioning at 50%, and Skikda has yet to start up.


Algeria quickly became the second leading producer of helium. Through this time, both helium consumption and the costs of producing helium increased and during 2007 the major suppliers, Air Liquide, Airgas and Praxair all raised prices from 10 to 30%.


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Occurrence and production

Natural abundance

Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of the elemental mass of the universe. It is concentrated in 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 during Big Bang nucleo-synthesis, from one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models.

In the Earth’s atmosphere, the concentration of helium by volume is only 5.2 parts per million. The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth’s atmosphere escapes into space by several processes. In the Earth’s heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.

Nearly all helium on Earth is a result of radioactive decay. The decay product is primarily found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite and monazite, because they emit alpha particles, which consist of helium nuclei (He2+) to which electrons readily combine. In this way an estimated 3.4 litres of helium per year are generated per cubic kilometre of the Earth’s crust. In the Earth’s crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived.

The world’s helium supply may be in danger, according to Washington University in St. Louis chemist Lee Sobotka. The largest reserve is in Texas and would run out in eight years if consumed at the current pace. Helium is non-renewable and irreplaceable by conventional methods.

Modern extraction

For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure, Grade-A, helium. The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.

In 2005, approximately one hundred and sixty million cubic meters of helium were extracted from natural gas or withdrawn from helium reserves, with approximately 83% from the United States, 11% from Algeria, and most of the remainder from Russia and Poland. In the United States, most helium is extracted from natural gas in Kansas and Texas.

Diffusion of crude natural gas through special semi permeable membranes and other barriers is another method to recover and purify helium. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this is not an economically viable method of production.


Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. In the Earth’s atmosphere, there is one He-3 atom for every million He-4 atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth’s crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.

The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.

Evaporative cooling of liquid helium-4, in a so-called 1-K pot, cools the liquid to about 1 kelvin. In a helium-3 refrigerator, similar cooling of helium-3, which has a lower boiling point, reaches a temperature of about 0.2 kelvin. Equal mixtures of liquid helium-3 and helium-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.

There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, a product of nuclear fusion. Extra-planetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon’s surface contains helium-3 at concentrations on the order of 0.01 ppm. A number of people, starting with Gerald Kulcinski in 1986, have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.

The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth’s mantle.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are hyper-fragments that are created in certain nuclear reactions.

The exotics helium-6 and helium-8 are known to exhibit a nuclear halo.

Helium-2 (two protons, no neutrons) is a radioisotope of helium that decays by proton emission into protium (hydrogen) with a half-life of 3×10−27 second.

Biological effects

The voice of a person who has inhaled helium temporarily sounds high-pitched. This is because the speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. (The opposite effect, lowering frequencies, can be obtained by inhaling sulphur hexafluoride).

Inhaling helium, e.g. to produce the vocal effect, can be dangerous if done to excess since helium is a simple asphyxiant, thus it displaces oxygen needed for normal respiration. Death by asphyxiation will result within minutes if pure helium is breathed continuously. In mammals (with the notable exceptions of seals and many burrowing animals) the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen, so asphyxiation by helium progresses without the victim experiencing air hunger. Inhaling helium directly from pressurized cylinders is extremely dangerous as the high flow rate can result in barotrauma, fatally rupturing lung tissue.

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen ( heliox) can lead to high pressure nervous syndrome, a sort of reverse anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.

Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.


Helium is chemically un-reactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulphur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He22+, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000.

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium NMR spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral helium compounds to be formed.

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From earliest times the rainbow had delighted and puzzled observers. Men invented myths to explain the beautiful arc of multicoloured light that appeared after the rain. But a scientific answer to the puzzle of the rainbow did not come until 1666. In that year Sir Isaac Newton began investigating the problem of eliminating the colour fringes in telescope lenses. (Scientists now call these colour fringes chromatic aberration.) He decided that the trouble might lie in the character of light itself. So he began to study how light formed colours

He admitted a small beam of sunlight into a darkened room and passed it through a prism. The beam produced a band of colours just like the rainbow, ranging from red through yellow, green, and blue to violet. He then passed each of these colours through other prisms and found that they did not change. But when he passed the whole band of coloured lights through a prism in reverse position, the coloured band became white sunlight again.

From this he reasoned that white light is really a mixture of coloured lights, and that each colour is bent by a different amount when it passes through the prism. This difference in bending enables each colour to stand out separately and be visible. The band of coloured lights thus formed is called a spectrum. The rainbow is actually a spectrum, formed by sunlight passing through raindrops.

Separating light into its colours is called dispersion. It is accomplished by refraction (bending) of light in the prism. Each of the colours has its own wavelength. The wavelength determines how much each colour will bend. Red bends the least, violet the most. If the light beam strikes the prism at a certain angle, the amount of bending for each colour is always the same. Each colour then falls in exactly the same place on a screen, so its position is enough to identify it.

Scientists use the dispersive action of the prism in the spectroscope. The spectroscope reveals that the spectral pattern of light is different for various classes of light sources. Light from the sun, from certain lamp filaments, and from molten metals each produces a spectrum which has all colours in an unbroken array. Such a pattern is called a continuous spectrum. Incandescent gases give off only certain colours, in fine lines. Their spectra are called bright-line spectra. Both bright-line and continuous spectra are emission spectra, produced by emitted light.

Fraunhofer, Joseph von (1787-1826), German optician and physicist; worked to improve optical instruments; invented a heliometer and a micrometer.

In the early 1800s Joseph von Fraunhofer observed that the continuous spectrum was crossed by many dark lines. He charted more than 700 of them, but he was unable to explain their meaning. Because of his discovery, however, they are called Fraunhofer lines.

Kirchhoff, Gustav Robert (1824-87), German physicist, born in Konigsberg, East Prussia (now Kaliningrad, Russia); developed spectrum analysis and discovered caesium and rubidium (with Bunsen); explained the Fraunhofer lines; professor of physics at Heidelberg 1854-74, at Berlin 1874-87.

The meaning of the Fraunhofer lines was discovered about 50 years later by Gustav Kirchhoff and Robert Bunsen. With a spectroscope they studied the spectra of certain substances which were vaporized in the non luminous Bunsen burner flame. Each vapour showed a characteristic bright-line spectrum. But when emitted light was passed through a cooler vapour of the same substance, the bright lines were replaced by dark ones in the same position.

This replacement of bright by dark lines meant that the second vapour had absorbed the characteristic light of the first. Later experiments showed that the cooler vapour absorbs those light waves which it would normally emit at a higher temperature. In 1859 Kirchhoff published his findings in his laws of radiation and absorption. The spectral pattern thus formed is called a dark-line, or absorption, spectrum.

Kirchhoff and Bunsen also noticed that characteristic arrays of lines are given off by the different chemical elements. For example, incandescent sodium always gives certain yellow lines near the middle of the spectrum, and no other element gives these lines. Thus when these lines appear, sodium must be present in the incandescent substance. If the lines are bright the light has come directly from the incandescent sodium. If they are dark the light has passed, somewhere along its path, through an absorbing vapour containing some gaseous sodium. Only minute quantities of an element are needed to make its lines appear. This makes it possible to identify the elements in unknown substances.

These discoveries not only explained the Fraunhofer lines in the spectrum of sunlight but made it possible to determine what chemical elements the sun contains. The absorption necessary to produce the dark lines was considered as taking place in the outer layers of incandescent gas surrounding the sun. For “analysis” of the sun, the dark lines could be compared with the bright-line spectra of different elements produced in the laboratory. Whenever they corresponded, scientists could be sure that the element existed in the sun. Stars likewise could be “analysed” as to chemical contents by this method.

Janssen, Pierre-Jules-Cesar (1824-1907), French astronomer, born in Paris; discoverer of helium in sun; founded and directed observatory on Mont Blanc 1893.

Lockyer, Joseph Norman (1836-1920), British astronomer and physicist, born in Rugby, England; pioneer in application of spectroscope to sun and stars; explained sunspots; between 1870 and 1905 conducted eight British expeditions for observing total solar eclipses (‘The Sun’s Place in Nature’; ‘Recent and Coming Eclipses’; ‘The Chemistry of the Sun’; ‘Inorganic Evolution’).

Ramsay, William (1852-1916), British chemist, born in Glasgow, Scotland; professor Bristol University 1880-87, University of London 1887-1913; discoverer of helium, neon, krypton, xenon; co-discoverer of argon; research in radioactivity led to new theory of transmutation of elements; knighted 1902; received 1904 Nobel prize.

Cleveite, mineral, oxide of uranium and lead; named for Per Theodor Cleve, a Swedish chemist; produces helium when heated with acid.

Scientists have obtained spectra corresponding to the different elements and have measured and charted every line. When they wish to learn the composition of a star, they photograph its spectrum and then check the lines against these charts for the elements. A notable triumph of the method was the discovery of helium. In 1868 P.J.C. Janssen (1824-1907), a French astronomer, and the English astronomer, Sir Norman Lockyer (1836-1920), independently discovered lines in the solar spectrum which could not be identified with the charted lines of any known element. Lockyer interpreted this to mean that an element unknown to us existed in the sun. He named it helium, after helios, Greek for “sun.” Then in 1895 Sir William Ramsay (1852-1916) found that the Norwegian mineral cleveite, when heated, gave off minute quantities of a light gas which he identified as helium by means of its spectrum.

Measuring Light Waves

Millimicron (mm), unit of wave length equal to one millionth of a millimetre or one thousandth of a micron; sometimes used in the measurement of light waves.

Angstrom unit (A, or A.U.), ten-millionth of a millimetre, used to describe length of light waves; named to honour Anders Jonas Angstrom.

Nanometre (nm), measuring unit in spectroscopy, equalling one billionth of a meter.

The units once commonly used to measure wavelengths were the millimicron, denoted by the symbol mm and equalling one millionth of a millimetre; and the Angstrom unit (A or A.U.), one ten millionth of a millimetre Wavelengths are still measured in various units, but the unit most commonly used in spectroscopic work today is the nanometre (nm), which is equal in length to the unit it replaced, the millimicron. This is one of the special units which science has accepted as a means of avoiding the excessively long decimal fractions which would be needed to express wavelengths as short as those of light, if measured in inches or centimetres For example, violet light has a wavelength of 410 nanometres. The following table gives the wavelengths which fall approximately in the centre of each of the coloured regions in visible light:

Violet 410 nanometres

Blue 470

Green 520

Yellow 570

Orange 620

Red 710

Since the colour of light is determined by its wavelength, this means that the shorter the wavelength the more the light is bent by passage through a given prism. Thus the wavelength (and the frequency) of the vibration causing the wave is judged from the amount of bending given by the prism. This is determined by the position of the spectral line on the screen or photographic plate.

Prism and Diffraction-Grating Spectroscopes

Modern spectroscopes used in very technical fields vary considerably in function and design and are often quite specialized for the specific substances they analyse However, the fundamental teaching instruments generally used in today’s classrooms are the relatively simple prism spectroscopes. These consist of a collimator (tube for admitting light), a glass prism, and a telescope. The collimator has a slit at one end to admit light and a lens on the other to concentrate it. The lens directs the light on the prism, which disperses the ray into its component colours Sometimes a train of prisms is used to increase the dispersion.

After the colours leave the prism they are focused on the object glass of the telescope. Each wavelength appears as a separate image of the collimator slit. When the telescope is replaced by a camera to photograph the lines, the device is called a spectrograph.

Rowland, Henry Augustus (1848-1901), U.S. physicist, born in Honesdale, Pa.; professor Johns Hopkins University 25 years; determined ohm and the mechanical equivalent of heat; discovered magnetic effect of electric convection.

A more powerful type of spectroscope uses a diffraction grating, invented by Fraunhofer in 1821. He made it by twisting a fine wire about two tiny screws. With it he measured the wavelengths of light with surprising precision. The modern precision grating consists of a plate of speculum metal or glass upon which fine lines, equidistant and parallel, have been ruled. Among the finest of these are the gratings made by H. A. Rowland. He invented a machine to rule the entire grating automatically, etching from 14,000 to 20,000 lines per 1 inch (2.5 centimetres). By means of such a grating, made on a concave surface, Rowland secured a spectrum band of sunlight more than 20 feet (6 meters) long. The grating uses a special application of the interference phenomenon of light.

Diffraction-grating spectroscopes can measure the wavelength of light with a precision of .000,000,000,001 centimetre (10-12 centimetre). It is used as the dispersing medium in analysing visible light and ultraviolet rays. A photographic plate is usually used as the detecting device in the analysis.

Motion, Temperature, Magnetism

Doppler effect, law in physics discovered by Christian Doppler (1803-53); applied to sound, light, and radar from moving sources.

The spectroscope can also tell the astronomer whether a star is moving toward or away from the Earth by means of a phenomenon known as the Doppler effect. Everyone has noticed how the whistle of an approaching locomotive rises to a shrill note as it approaches then drops to a lower and lower tone as the train rushes away. The reason for this is that when the train approaches, its whistle is nearer to us each time a sound wave is emitted. The successive waves reach us a little more quickly and therefore have a higher pitch. When the train is receding, the waves are dragged out, and thus the pitch of the whistle is lowered.

Similarly, when a star moves toward the Earth, each light wave is shortened a little. Consequently, the lines shift their position toward the violet end of the star’s spectrum. When the star is moving away from the Earth, the wavelengths are lengthened somewhat, and the lines in the spectrum shift a little toward the red end. The amount of shift reveals the speed of the star’s motion; but since light in a vacuum travels at the tremendous speed of 186,282 miles per second (299,743 kilometres per second), the star must be travelling at a very great speed to create a noticeable effect.

Temperature, degree of hotness or coldness measured on a definite scale.

Temperature and pressure have certain effects on spectra. These effects can be detected and used to determine the approximate temperature of stars and the pressure of gases on distant bodies.

Zeeman, Pieter (1865-1943), Dutch physicist; professor physics and director Physical Institute, University of Amsterdam, 1900-35; discovered the Zeeman effect of magnetism on light; Nobel prize 1902.

Another marvellous revelation of the spectroscope is the connection between magnetism and light. In 1896 the Dutch physicist Pieter Zeeman (1865-1943) discovered that when light passed through the field of a strong electromagnet, the lines in the resulting spectrum were split into two or more lines. This influence of magnetism on light, which was named the Zeeman effect after its discoverer, has proved to be valuable in the detection and measurement of magnetism in the sun.

The Electromagnetic Spectrum

The coloured lights in the rainbow make up but a small portion of that huge spectrum of energy called electromagnetic radiation. The other groups include radio waves, microwaves, infra red light (heat), ultraviolet rays, X rays, and gamma rays. Despite the different effects they produce, each of these forms of energy travels through space as an electromagnetic disturbance. They are sometimes called forms of radiant energy.

Posted 2012/01/08 by Stelios in Education

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