NOBLE GAS   Leave a comment

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.

Etymology

“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.

History

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.

Applications

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.

Synopsis from: https://en.wikipedia.org/wiki/Noble_gas

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Posted 2018/02/01 by Stelios in Education

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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.

 

Synopsis of: https://en.wikipedia.org/wiki/Helium

 

Posted 2018/01/25 by Stelios in Education

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Applications

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.

 

History

 

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%.

 

Synopsis of: https://en.wikipedia.org/wiki/Helium

 

Posted 2018/01/25 by Stelios in Education

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HELIUM 3 of 3   Leave a comment

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.

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 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.

Compounds

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.

Synopsis of: https://en.wikipedia.org/wiki/Helium

Posted 2018/01/25 by Stelios in Education

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TERMITES   Leave a comment

DEFINITION: any of an order (Isoptera) of pale-coloured social insects having a soft body and living in colonies composed of winged forms that mate and wingless workers and soldiers that are usually sterile or immature: they are very destructive to wooden structures and are found in the temperate zones and esp. in the tropics.

Although they are closely related to cockroaches, termites are sometimes called “white ants” because their general appearance and social organization are like those of the ants. Termites, however, are distinguished from ants by their soft bodies and lighter colour Ants have hard bodies and are usually dark. Furthermore, the termite’s mid body segment, or thorax, is broadly attached to the rear segment, or abdomen, whereas ants have a constriction where the thorax joins the abdomen. Termites belong to the insect order Isoptera.

Ground Nesters

More than 2,000 species of termite have been described, most of which live in the tropics. More than 40 species live in the United States. A typical colony lives underground in a damp, chamber like nest. The colony is organized into a caste system with four different adult forms: royalty, nobility, soldiers, and workers. The royalty consists of the kings and queens, which carry on the work of reproduction. They have well-developed wings and eyes. The kings are usually smaller than the queens, which may reach a length of 4.3 inches (11 centimetres) in some species.

Once a year pairs of young kings and queens depart from the parent nest, leaving the ruling king and queen behind. Each pair starts a new colony nearby. They then shed their wings. Within a short time the young queens may begin laying eggs at the rate of 3,000 to 5,000 a day. The nobility consists of wingless or short-winged adults. They take over the work of reproduction if a king or queen should die.

The soldiers and workers grey-white, wingless, usually blind, and less than 0.4 inch (1 centimetre) long are the most populous members of the colony. Both male and female soldiers and workers are sterile, so they cannot reproduce. The soldiers, which have large heads and jaws, guard the nest against insect enemies, chiefly ants. The workers keep the colony supplied with food, and they actually feed the queens, soldiers, and young termites.

Cellulose, complex carbohydrate consisting of 3,000 or more glucose units; basic structural component of plant cell walls; 90% of cotton and 50% of wood is cellulose; most abundant of all naturally occurring organic compounds; indigestible by humans; can be digested by herbivores, such as cows and horses, because they retain it long enough for digestion by micro-organisms present in their digestive systems; also digestible by termites; processed to produce papers and fibres; chemically modified to yield plastics, photographic film, and rayon; other derivatives used as adhesives, explosives, thickening agents, and in moisture-proof coatings.

Termites feed primarily on wood fibre, or cellulose, which they get from dead trees, rotting plant material in the soil, fence posts, house timbers, or furniture. Although some kinds of termites can destroy human dwellings, they serve a vital function in the food web by recycling the nutrients in dead wood so that the nutrients can be used by bacteria and plants.

Cellulose is indigestible to nearly all animals, large or small, including termites. The termite workers, however, have formed a remarkable partnership, or symbiosis, with micro-organisms called protozoans. The workers harbour the protozoans in their intestines. As they chew and swallow the wood fibre, the protozoans transform it into a product that termites can digest. Soldiers also have symbiotic protozoa, and they can digest cellulose after the workers have chewed it up for them. The soldiers’ enormous fighting jaws prevent them from gathering this fibre for themselves. Royalty and nobility lack protozoans and are fed on digested cellulose secreted by workers.

Worker termites may eat wood that is above ground by entering the wood where the timbers touch the ground. If a house has a stone foundation, the termites may build tubular, earthen passages over the foundation and up to the house beams. The termites thus maintain their contact with the ground and the necessary moisture. Under a porch they may erect towers more than 1 foot (0.3 meter) high to reach the wooden floor. Once inside the woodwork of a building, they tunnel in all directions, with no openings showing on the surface. Houses may be inspected for signs of termite problems by searching for hollow timbers, termite nests at the base of wood, or the insects themselves. Unfortunately, the first sign of their presence may be the collapse of a wall or some other wooden structure. Termites work in large numbers as many as 4,000 have been counted in 1 cubic foot (0.03 cubic meter) of wood.

To rid an area of termites would require the destruction of all the nests. It is more practical to “insulate” a building against the insects by treating woodwork with chemicals or by covering all possible points of attack with metal.

Ground-nesting termite, a termite (Reticulitermes flavipes) of the order Isoptera.

The scientific name of the common ground-nesting termite of eastern North America is Reticulitermes flavipes. Besides the ground-nesting termites there are dry wood, damp wood, and powder post termites. Many of these species live not in soil but in the wood that they attack. They do not require moisture from the soil because they can conserve water in their bodies. These termites can also be eliminated from infested sites by the use of chemicals.

Mound Builders

Mound-building termites live in South America, Africa, and Australia. Their brown mounds, or termitaries, often crowd together in a close group of slender towers. They are built of saliva-soaked soil particles and are as hard as concrete. Some termitaries are decades old and are more than 23 feet (7 meters) high and 43 feet (13 meters) wide at the base. The bases of some termitaries are oval, with the long axis pointing north and south, presumably so that the sun can reach both of the broad outside walls and keep them warm and dry.

Inside the walls of each termitary the social order is the same as that of the ground-nesting termites. The king and queen occupy the royal chamber. The king is small, but the queen is large and may carry as many as 75,000 eggs. Some of the larger queens lay one egg each second, 24 hours a day, during their reproductive life, which may last up to 10 years. After being laid the eggs are taken by nurses, washed with saliva to prevent mould, then carried to the hatchery, which is kept warm by decaying vegetation.

The sightless soldiers, with their strong scissors-like mandibles, guard every turn of the galleries inside the nest. Other soldiers, equipped with tough “helmets” to check any onrush of ants, guard the entrances from the outside world. The soldiers of some species have snouts through which they spray a sticky liquid that entangles the legs of their enemies and that also stupefies them. The worker caste gathers bits of wood to feed the entire community. Some termite colonies grow small mushrooms in fungus gardens for their food. Some have community “cows” small beetles called termitophiles that live only in termite nests and secrete a fluid relished by the termites.

Assisted by J. Whitfield Gibbons

Posted 2012/09/08 by Stelios in Education

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RAINBOWS   Leave a comment

DEFINITION: an arc or ring containing the colours of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog.

When light from a distant source, such as the sun, strikes a collection of water drops such as rain, spray, or fog a rainbow may appear. It appears as a multicoloured arc whose “ends” seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun. The centre of the rainbow’s arc is located on an imaginary line extending from the light source through the observer’s eye to the area of the water drops.

Rainbows are most commonly seen when the sun’s rays strike raindrops falling from distant rain clouds. Generally, this is only in the early morning or late afternoon. When the sun is too far above the horizon no rainbow can be seen.

When the sun is lower in the sky, however, part of the arc becomes visible. In fact, if the sun is low enough and the observer is located in a place that is high enough, such as on a mountain, in an aeroplane or a spaceship, the observer may see a circular rainbow.

The most brilliant and most commonly seen rainbow is called the primary rainbow. The arcs of colour in a rainbow are caused by the refraction, or bending, and internal reflection of light rays that enter the raindrops. A ray of white sunlight is actually composed of all the colours of the spectrum. Inside the drop the ray of white light is separated into the colours that make it up and reflected back toward the observer. In the primary rainbow the colours are, from inside to outside, violet, blue, green, yellow, orange, and red. The red band makes an angle of about 42 degrees with the sun’s rays, and the other coloured bands make successively smaller angles. Sometimes another less intense rainbow may also be seen; this is called the secondary bow. The secondary bow, when visible, is seen outside the primary bow and with its colour sequence reversed. It is produced by light that has been reflected from two different points on the back of the drop before emerging into the air. Higher-order rainbows are very weak and so are rarely seen.

Occasionally, faintly coloured rings are seen just inside the primary bow. These are called spurious, or supernumerary, bows. When raindrops are extremely fine, an almost white bow, called a fog bow, is produced. A fog bow at night, sometimes called a lunar rainbow, is made by sunlight reflected from the moon and appears as a ring around the moon.

Posted 2012/08/26 by Stelios in Education

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CATERPILLARS   Leave a comment

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DEFINITION: the worm like larva of various insects, esp. of a butterfly or moth [C-] trademark for a tractor equipped on each side with a continuous roller belt over cogged wheels, for moving over rough or muddy ground.

The larvae, or young, of butterflies and moths are called caterpillars, from the Latin catta pilosa, meaning “hairy cat.” Although people usually recognize the hairy kinds, many caterpillars with bare skins are popularly called worms, such as the cabbage worm and army worm

A caterpillar’s body consists of a head followed by 12 or 13 segments. Like all insects, it also has three pairs of permanent or “true” legs one pair on each of the first three segments directly behind the head. These true legs are usually hard, jointed, and tipped with tiny claws, but in a few caterpillars they are not developed. To support the rest of its long body, the caterpillar also has from two to five pairs of soft, thick prolegs that disappear when it changes into a moth or butterfly.

A caterpillar has six eyes like tiny beads on each side of the head, just above the strong upper jaws. It breathes through nine pore like openings, called tracheae, on each side of its body.

When a caterpillar hatches from the egg laid by a female butterfly or moth, it is usually very small. But it grows rapidly and soon gets too large for its skin. Thereupon the old skin splits, and the caterpillar wriggles out of it, revealing a soft new covering. This skin-shedding is called moulting and occurs four or five times. Some caterpillars eat their old skins. The hawk moth caterpillar, one of the largest, may grow 4 inches (10 centimetres) long; the clothes moth caterpillar, one of the smallest, seldom exceeds a quarter of an inch. Some caterpillars may take only a few days before they turn into butterflies or moths, but most last throughout the warm season. A very few may live as long as four years in the caterpillar form before they change.

Cocoon, envelope, often largely of silk, which an insect larva forms around itself.

The change that caterpillars undergo is called metamorphosis. The first step for many moth caterpillars is to build cocoons. They spin them with threads of sticky fluid that flows from an opening in the lower lip and hardens in the air.

Some caterpillars form bags of silk that entirely enclose them. Others roll up a leaf, fastening the edges with the silk. Many of the hairy kinds pad the cocoons with their own hair.

Some caterpillars do not build cocoons. Many of the moth caterpillars take shelter simply by burrowing in the ground or under a stone or fallen leaf. Butterfly caterpillars may suspend themselves from leaves or twigs by their tails, or spin a button of silk on a twig or leaf and hang from it by a silk girdle.

Pupa, quiescent stage between larva and adult in insect metamorphosis.

Whether protected by a cocoon or not, the caterpillar becomes ready to shed its last skin, and in place of it grows a tough flexible shell or case. When this happens it has become a pupa. The moth pupa is usually dull brown and mummy like The butterfly pupa, sometimes called a chrysalis, is shiny and often brilliantly coloured

Inside the pupal, or chrysalids, case, the rudimentary wings and other organs enlarge to make the moth or butterfly. This transformation from the larval to the adult stage may be completed in a few days or take several months.

To grow and prepare for this period of change, caterpillars eat enormously, causing widespread damage to trees, flowers, and crops. The larva of the Polyphemus moth, a species of the American silkworm, has been estimated to eat as much as 86,000 times its own weight during its 56 days as a caterpillar.

Caterpillars are the prey of many birds and insects, especially parasites. To avoid their attacks, caterpillars have various natural protections. Some are coloured to blend with their surroundings. Others have gaudy dots or stripes to make them look fierce or very large. A few give off unpleasant smells, and a very few grow poisonous nettle like hairs.

Posted 2012/08/19 by Stelios in Education

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