Helium

2 hydrogen ← helium → lithium - ↑ He ↓ Ne periodic table
General
Name, Symbol, Number	helium, He, 2
Chemical series	noble gases
Group, Period, Block	18, 1, s
Appearance	colorless
Atomic mass	4.002602(2) g/mol
Electron configuration	1s2
Electrons per shell	2
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 0.1786 g/L
Melting point	(at 2.5 MPa) 0.95 K (-272.2 °C, -458.0 °F)
Boiling point	4.22 K (-268.93 °C, -452.07 °F)
Critical point	5.19 K, 0.227 MPa
Heat of fusion	0.0138 kJ/mol
Heat of vaporization	0.0829 kJ/mol
Heat capacity	(25 °C) 20.786 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K         3 4
Atomic properties
Crystal structure	hexagonal or bcc
Ionization energies	1st: 2372.3 kJ/mol
	2nd: 5250.5 kJ/mol
Atomic radius (calc.)	31 pm
Covalent radius	32 pm
Van der Waals radius	140 pm
Miscellaneous
Thermal conductivity	(300 K) 151.3 mW/(m·K)
CAS registry number	7440-59-7
Notable isotopes
Main article: Isotopes of helium iso NA half-life DM DE (MeV) DP 3He 0.000137%* He is stable with 1 neutron 4He 99.999863%* He is stable with 2 neutrons *Atmospheric value, abundance may differ elsewhere.

For other uses, see Helium (disambiguation).

Helium (from Greek Hêlios: god of the Sun) (chemical symbol He, atomic number 2) is the second most abundant element in the universe and second lightest of all known elements. It is a colorless, odorless, tasteless, nontoxic, and nearly inert gas that heads the noble gas series in the periodic table. Its boiling and melting points are the lowest among the elements. It exists 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. Its most abundant stable isotope is helium-4 and it has a rare stable isotope, helium-3.

In the present-day universe, almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth, it is created by the radioactive decay of much heavier elements (alpha particles are helium-4 nuclei produced by alpha-decay). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation.

Helium 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). Inhaling a small volume of the gas temporarily changes the quality of one's voice. However, inhaling it from a typical commercial source, such as that used to fill balloons, can be dangerous due to the number of contaminants that may be present. These could include trace amount of other gases, in addition to aerosolized lubricating oil.

Abundance in nature

Helium is the second most abundant element in the known universe, after hydrogen, constituting 23% of the elemental mass of the universe. It is concentrated in stars, where it is formed by two sets of nuclear fusion reactions: one involving the "proton-proton chain reaction" and the other involving the "carbon-nitrogen-oxygen cycle." According to the Big Bang model of the early development of the universe, the vast majority of helium was formed between one and three minutes after the Big Bang, at a stage known as the Big Bang nucleosynthesis. Based on this view, measurements of the abundance of helium contribute to cosmological models.

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million, largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. 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 found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl. These minerals emit alpha particles, which consist of helium nuclei (He²⁺), to which electrons readily attach themselves. In this way, an estimated 3.4 liters of helium are generated per year per cubic kilometer of the Earth's crust.

The concentration of helium in the Earth's crust is 8 parts per billion; in seawater, it is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations of helium on our planet are in natural gas, from which most commercial helium is derived.

Scientific discoveries

Pierre Janssen (1824–1907), a French astronomer, was the first to detect evidence of a previously unknown element (helium) in the Sun.

On August 18, 1868, during a total solar eclipse in Guntur, India, French astronomer Pierre Janssen observed a bright yellow line with a wavelength of 587.49 nanometers (nm) in the spectrum of the Sun's chromosphere. This line was the first evidence that the Sun contained a previously unknown element, but Janssen was ridiculed because no element had been detected in a celestial body before being found on Earth. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line of the same wavelength in the solar spectrum. He named it the D₃ line, for it was near the known D₁ and D₂ lines of sodium. He 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, ἥλιος (helios).

On March 26, 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 sulfuric acid, he 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. That same year, chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden independently isolated helium from cleveite. They collected enough of the gas to accurately determine its atomic weight.¹

In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle (emitted by radioactive materials) is a helium nucleus. In 1908, Dutch physicist Heike Kamerlingh Onnes was the first to liquefy helium by cooling the gas to below 1 Kelvin (K). He tried to solidify it by further reducing the temperature, but he failed because helium does not have a "triple point" temperature where the solid, liquid, and gas phases are in equilibrium with one another. His student, Willem Hendrik Keesom, was the first to solidify helium in 1926, by subjecting it to a pressure of 25 atmospheres.

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 with helium-3, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.

Notable characteristics

Gas and plasma phases

Helium is a colorless, odorless, tasteless, and nontoxic gas. It is the least reactive member of group 18 (the noble gases) of the periodic table and is virtually inert. Under standard temperature and pressure, helium behaves very much like an ideal gas. Under virtually all conditions helium is monatomic. It has a thermal conductivity that is greater than any gas except hydrogen and its specific heat is unusually high. Helium is also less water soluble than any other gas known and its diffusion rate through solids is three times that of air and around 65% that of hydrogen. Helium's index of refraction is closer to unity than any other gas. This gas 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 precooled below this temperature, helium can be liquefied through expansion cooling.

File:HeTube.jpg

Helium discharge tube shaped like the element's atomic symbol

Helium is chemically unreactive 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, sulfur, 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⁺, He₂⁺⁺, 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, like helium fluorohydride (HHeF), may also be possible.

Throughout the Universe, helium is found mostly in a plasma state whose properties are quite different to molecular helium. As 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 colorless, almost invisible solid is highly compressible; applying pressure in the laboratory can decrease its volume by more than 30%. With a bulk modulus on the order of 5×10⁷ Pa [1] 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. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 26 standard atmospheres (2.6 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.

The behavior of liquid helium-4's two different states—helium I and helium II—is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and those looking at the effects that near absolute zero temperatures have on matter (such as superconductivity).

Helium I state

Below its boiling point of 4.21 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when heat is added to it. 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 colorless liquid has a very low viscosity and a density 1/8th that of water, which is only 1/4th 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 superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3.

Helium II will "creep" along surfaces in order to find its own level - after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.

Helium II is a superfluid, 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 superfluid 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, B. V. Rollin. As a result of this creeping behavior 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.

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium in order to maintain the equilibrium fraction of superfluid helium. Superfluid 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.

Isotopes

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. 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, and its abundance serves as a test of cosmological models.

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. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds.

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 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 hyperfragments that are created in certain nuclear reactions.

Applications

Because of its low density, helium is the gas of choice to fill airships such as this USGS blimp.

Helium is used for many purposes that require some of its unique properties, such as low boiling point, low density, low solubility, high thermal conductivity, or inertness:

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

• For its low solubility in water, air mixtures of helium with oxygen and nitrogen (Trimix), with oxygen only (Heliox), and with hydrogen and oxygen (hydreliox), are used in deep-sea breathing systems to reduce the high-pressure risk of nitrogen narcosis, decompression sickness, and oxygen toxicity.

• For its extremely low melting and boiling points, helium is used as a coolant in magnetic resonance imaging, superconducting magnets, and cryogenics.

• For its inertness and high thermal conductivity, helium is used as a coolant in some nuclear reactors, such as pebble-bed reactors, and in arc welding.

• 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 oxidizers 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 precool liquid hydrogen in space vehicles..

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

• For its extremely low temperature, liquid helium is used to produce superconductivity in some ordinary metals, such as lead, allowing for a completely free flow of electrons in the metal.

• Because it diffuses through solids at a rate three times that of air, helium is used in detecting leaks in high-vaccuum equipment and high-pressure containers.

• 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.

Production and use

After an oil-drilling operation in 1903 in Dexter, Kansas, 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. There, 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.² With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium.³ 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 (5,700 m³) of 92% helium was produced in the program even though only a few cubic feet (less than 100 liters) 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 December 7, 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 Manhattan Project that produced the atomic bomb.

In 1925, the U.S. government set up the National Helium Reserve at Amarillo, Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime. 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 when creating 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 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 meters 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.⁴ The resulting "Helium Privatization Act of 1996" (Public Law 104–273) directed the U.S. 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 early 2000s, Algeria and Qatar were added as well. Algeria quickly became the second leading producer of helium. Through this period, helium consumption and costs have increased.

Precautions

The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters Alvin and the Chipmunks (although their voices were produced by shifting the pitch of normal voices). This is because the speed of sound in helium is nearly three times that in air. As a result, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. The higher perceived pitch is only due to a different frequency shaping of the voice, the fundamental frequency of the vocal cords remains more or less the same.

Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess. The reason is not due to toxicity or any property of helium but simply due to it displacing oxygen needed for normal respiration. One must be aware that in mammals (with the notable exception of seals) the breathing reflex is not triggered by insufficient oxygen but rather excess of carbon dioxide. Unconsciousness, brain damage and even asphyxiation followed by death may result in extreme cases. Also, if helium is inhaled directly from pressurized cylinders the high flow rate can fatally rupture 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, a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome; a small proportion of nitrogen can alleviate the problem.

Containers of helium gas at 5 to 10 K should be treated as if they have liquid inside. This is due to the rapid and large increases in pressure and, if allowed, volume that occur when helium gas at that temperature is warmed to room temperature.

References

ISBN links support NWE through referral fees

Prose

Specific references are indicated by comments in the article source

• The Encyclopedia of the Chemical Elements, edited by Cifford A. Hampel, "Helium" entry by L. W. Brandt (New York; Reinhold Book Corporation; 1968; pages 256-267) Library of Congress Catalog Card Number: 68-29938
• Emsley, John. Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2001. Pages 175–179. ISBN 0-19-850340-7
• Los Alamos National Laboratory (LANL.gov): Periodic Table, "Helium" (viewed 10 October 2002 and 25 March 2005)
• Guide to the Elements: Revised Edition, by Albert Stwertka (New York; Oxford University Press; 1998; pages 22-24) ISBN 0-19-512708-0
• The Elements: Third Edition, by John Emsley (New York; Oxford University Press; 1998; pages 94-95) ISBN 0-19-855818-X
• United States Geological Survey (usgs.gov): Mineral Information for Helium (PDF) (viewed 31 March 2005)
• The thermosphere: a part of the heterosphere, by J. Vercheval (viewed 1 April 2005)
• Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements, Zastenker G.N. et al., [2], published in Astrophysics, April 2002, vol. 45, no. 2, pp. 131-142(12)
• Dynamic and thermodynamic properties of solid helium in the reduced all-neighbours approximation of the self-consistent phonon theory, C. Malinowska-Adamska, P. Sŀoma, J. Tomaszewski, physica status solidi (b), Volume 240, Issue 1 , Pages 55 - 67; Published Online: 19 September 2003
• The Two Fluid Model of Superfluid Helium, S. Yuan, (viewed 4 April 2005)
• Rollin Film Rates in Liquid Helium, Henry A. Fairbank and C. T. Lane, Phys. Rev. 76, 1209–1211 (1949), from the online archive
• Introduction to Liquid Helium, at the NASA Goddard Space Flight Center (viewed 4 April 2005)
• Tests of vacuum VS helium in a solar telescope, Engvold, O.; Dunn, R. B.; Smartt, R. N.; Livingston, W. C.. Applied Optics, vol. 22, 1 January 1983, p. 10-12.
• Bureau of Mines (1967). Minerals yearbook mineral fuels Year 1965, Volume II (1967). U. S. Government Printing Office. 
• Helium: Fundamental models, Don L. Anderson, G. R. Foulger & Anders Meibom (viewed 5 April 2005)
• High Pressure Nervous Syndrome, Diving Medicine Online (viewed 5 April 2005)

Table

• Nuclides and Isotopes Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989
• WebElements.com and EnvironmentalChemistry.com per the guidelines at Wikipedia's WikiProject Elements (viewed 10 October 2002)

Notes

• Note 1: Emsley, Nature's Building Blocks, p. 177

• Note 2: Emsley, Nature's Building Blocks, p. 179

• Note 3: American Chemical Society (2004). The Discovery of Helium in Natural Gas URL accessed on 2006-05-19.

• Note 4: Emsley, Nature's Building Blocks, p. 179

External links

General

• WebElements: Helium
• It's Elemental – Helium
• Photos and applications of Helium
• Fluidmech.net about liquid Helium-II and low temperature phase diagram

More detail

• Helium at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4.

Miscellaneous

• Helium Safety regarding inhalation
• Physics in Speech with audio samples that demonstrate the unchanged voice pitch
• Article about Helium and other noble gases
• this article contains phase diagram for helium

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