Helium

Helium He, Neon Ne, Argon A, Krypton Kr, Xenon Xe, Radon Rn


Contents

  1. Introduction
  2. The Source of Helium
  3. Why the Inert Gases are Inert
  4. The Inert Gases as Gases
  5. Xenon and Radon
  6. Liquid Helium
  7. Helium-Three
  8. Uses of Helium and Argon
  9. References

Introduction

This article is about the extraordinary gas helium, but at the same time it will be convenient to talk about the other members of its family in the periodic table. They are known collectively as the noble, inert or rare gases, because they do not form chemical compounds. They also are quite rare, and became known only at the end of the 19th century. An especially remarkable fact about helium is that it is available in large, industrial quantities, and was even used to fill airships. The availability of helium in such large quantities is an almost unbelievable surprise. One purpose of this article is to make clear how we get our helium.

In 1785, Henry Cavendish found that about 1/120 of the volume of air was a gas that was not oxygen, nitrogen or carbon dioxide, and which would not react chemically. This discovery, like others of Cavendish's, was largely forgotten. Indeed, Cavendish himself thought it might just be experimental error. On the 18th August 1868 there was a total eclipse of the sun visible from India. Pierre Janssen studied the chromosphere of the sun, which was until then only visible during total eclipses, with a spectroscope and found its spectrum to consist of bright lines, as are emitted by excited gases. One very prominent yellow line at 587.56 nm did not correspond to any known element. At the same time, Norman Lockyer finally perfected his spectroheliograph, which allowed the study of the chromosphere at any time, and he, too observed the mysterious yellow line. Lockyer made the leap to propose that this line corresponded to some element common on the sun, but rare on earth, and he called that supposed element helium. Helium does produce a Fraunhofer line in the solar spectrum, D3, but it is too close to the sodium D lines to be easily observed. A search for helium began, but nobody knew where to look.

The search for terrestrial helium was unavailing, and the source of the yellow line remained obscure. Then, after 1890, Rayleigh and Ramsay discovered that nitrogen produced chemically (by heating NH4NO3, for example) was lighter than nitrogen separated from air by fractional distillation of liquid air. Rayleigh recalled Cavendish's forgotten observation, and it set him thinking. Liquefaction of gases was a new science then, in which Kamerlingh Onnes was a pioneer. It turned out to help greatly in the search for helium. Ramsay separated the heavier component from atmospheric nitrogen in 1894, calling it argon. This name comes from Greek a-ergos (in which the "e" was elided, making the word identical to another Greek word meaning "brilliant"). This means "not-working" or lazy, inert or inactive, which perfectly describes argon's chemical interests. The -on is simply the neuter ending.

Ramsay heard of inert gases given off by uranium minerals in the studies of W. F. Hillebrand of the USGS published in 1891. The gas given off when uraninite was dissolved in acid was studied spectroscopically by Hillebrand and found to be apparently only nitrogen. The nitrogen spectrum had obscured the feeble yellow line from the small amount of helium present. Ramsay obtained some clevite, another uranium mineral that by chance was available, and treated it the same way. Taking care to eliminate any nitrogen impurity, he saw a strange spectrum, and named the gas he had found crypton. Some was sent to William Crookes, who identified the gas as the long-sought helium. Therefore, in 1895, terrestrial helium was finally found.

With argon and helium now identified, the periodic table suggested that there were other members of the family. Ramsay, with the help of Travers, went to work to see if they could be discovered in the argon separated from atmospheric nitrogen, by fractional distillation of the liquefied gases. In 1898, they announced the discovery of neon, krypton and xenon. Kayser found that helium was also present in air. Therefore, all of the gases were found in atmospheric air. Argon was present to about 1% by volume, a considerable amount, but the other gases were by far scarcer. In ppm by volume, air has the composition Ar, 9300; CO2, 300; Ne, 18; He, 5.2; Kr, 1.0; Xe, 0.8. Air is, therefore, a good source of argon in large volumes, but Ne, He, Kr and Xe can be separated from it only in small amounts and at high cost. These amounts would be sufficient for discharge tubes, which use only very small amounts, and for scientific purposes, but not for any wide-scale use.

In 1907, Rutherford showed that the alpha-particles emitted at high velocity from radioactive substances were helium nuclei. When alpha particles were allowed to enter a vacuum through a thin screen, the characteristic spectrum of helium was soon observed when an electrical discharge was produced. Most of the helium on the earth, in fact, was created in the form of alpha particles. The disintegration of radioactive substances has been going on throughout the existence of the earth. This radioactivity occurs mainly in the upper mantle, just beneath the crust, where the heavy atoms have been concentrated by convection. Helium that reaches the atmosphere is "boiled off" by the earth and is dissipated into space, just as hydrogen and other light gases are. Water, with a molecular weight of 18, is just heavy enough to have been largely retained (but it seems to have boiled off of smaller Mars already). Neon, with an atomic weight of 20, has been retained by gravity, as have, of course, the heavier xenon and krypton. On the other hand, the original abundance of these atoms on the earth decreases rather sharply as the atomic weight increases. The abundance of the noble gases in the atmosphere seems to be well accounted for by these considerations. Argon, it should be noted, is the product of the beta-decay of naturally-occurring K40, which may be the source of most of it. Although K40 is rare (1 part in 104), there is a lot of natural potassium.

The Source of Helium

The helium from most of the alpha particles, however, has been trapped deep within the earth, not far from the top of the mantle. Helium diffuses very rapidly, so it can be expected to have been mobile in the crust. The Great Plains of the United States consist of a thin crust of sedimentary rock on a basement of igneous rock that would be a source of radiogenic helium. The helium has collected where other gases have collected, depending on the chances of distribution. In some of these gases, helium is present up to about 8% by volume, a surprising amount. However, in most gases the abundance is less than 1%, sometimes much less, and the occurrence is very rare on a world-wide basis. Some natural gas or well helium, as radiogenic helium from the earth is also called, is found in the Canadian west, and, in lesser quantities, in gas associated with the blue rock salt of Germany, and a few other locations. There may also be helium in Siberia, where the geologic conditions resemble those of the Great Plains.

Porous and permeable rocks deep below the surface are often saturated with salt water, at a pressure of the same order as the hydrostatic pressure at that depth, called the formation pressure. Since 33 ft of water corresponds to 1 atm, or 14.7 psi, the formation pressure at 3300 ft would already be 1470 psi. Gases in permeable rocks collect at the highest points they can reach, and there form a "cap" of gas, possibly underlaid with liquid petroleum. A well drilled into the part of the formation containing liquid petroleum would soon fill with the liquid and flow out of the hole, as quickly as it could ooze into the hole. The weight of the petroleum would largely balance the formation pressure, so the oil would generally flow placidly. In rare cases, the formation pressure might be high enough to project a jet into the air, a "gusher." As formation pressure is lost, as it always is when holes are drilled, the oil might not reach the surface, and then it would have to be pumped out mechanically. Oil cannot be sucked from rocks; you can only collect what shows up.

With gas, what happens is quite different. The gas is so light that it cannot balance the formation pressure, and the gas exits from the hole at high pressure, unless confined (perhaps by the fluids used in drilling the well). Such a well was called a "blower" in early days, and the owners hardly realized that what was blowing out was, in effect, their wealth, since with depleted formation pressure little oil or gas could be produced. A gas well is exhausted when the pressure is no longer sufficient to bring the gas to the surface at sufficient pressure. These days, much gas in the western United States is produced from shallow coal-bed wells or from formations of small permeability, or "tight" formations. This gas has to be compressed before it can be put into the gas pipeline.

Natural gas is often thought of as methane, CH4, and most marketable natural gas is chiefly methane. However, the gas ranges from a rich gas containing considerable ethylene and even heavier components, which give it a high heating value, down to lean gases containg so much nitrogen that it will not even burn if ignited. The bacteria that produced the oil also leave behind carbon dioxide and hydrogen sulphide, which are often found in the gas. Hydrogen sulphide makes the gas poisonous and of foul odor; such gas is called "sour." The hydrogen sulphide is relatively easily removed, and the sulphur that results is a valuable by-product. Some gas is almost pure carbon dioxide, and many wells are produced for the carbon dioxide, a valuable chemical. Indeed, carbon dioxide is often pumped underground to repressurize a formation, and serve as a lubricant for getting the oil out. Sweet natural gas has little odor and is not poisonous.

Some of this natural gas also contains helium. Gas rich in helium usually also has considerable nitrogen, but much nitrogen does not imply helium. Helium was first found in a "blower" at Dexter, Kansas southeast of Wichita near Winfield in 1905. This gas would not burn, to the disappointment of the locals, but it did contain 1.84% helium, as H. P. Cady at Kansas University found. This was a chemical curiosity, since it had been expected that helium would be very rare, but helium had no uses at the time. This status changed completely during World War I with the introduction of Zeppelins. These rigid airships were filled with hydrogen, as indeed this was the only thing available other than hot air. Hydrogen diffused out of an inflated airship, while oxygen from the air diffused in. Not only was buoyancy lost, but more dangerously, an explosive mixture could be formed. When a light is touched to a balloon filled with pure hydrogen, it burns rapidly. When a light is touched to a balloon filled with an explosive mixture of hydrogen and oxygen (and the mixture is explosive over a wide range of concentrations) there is a violent catastrophe. In the well-known Hindenburg disaster, the hydrogen merely burned. If the mixture had been contaminated with oxygen, everything would have been blown to smithereens.

The lifting power of hydrogen is proportional to the difference in molecular weights between hydrogen and air, which is 29 - 2 = 27. The lifting power of helium is 29 - 4 = 25, which is 93% that of hydrogen. Replacing hydrogen with helium sacrifices little lifting power, but gains the immense advantage of not being flammable. After World War I, the great advantages of helium-filled dirigibles and barrage balloons were evident, so there was great military interest in helium. Without the Kansas helium, this would have been utterly impossible. Therefore, the development of helium became something of a military secret. Secrets are one of the pleasures of a military life, and they were enjoyed to the full over helium. Seibel (see References) tells the story of the development of helium for military purposes by the United States.

In Kansas, a thin layer of mainly Pennsylvanian rocks, in which there was enough organic activity to produce natural gas, probably by bacterial action, overlies granitic rocks of the continental shield. In fact, the Nemaha Ridge is a granite mountain trending north-south through central Kansas beneath the sediments, and apparently giving off helium as a result of its radioactivity. This midcontinent field was rich in crude oil, and especially in natural gas. Because natural gas was difficult to transport economically (it is too light), it was mostly considered a waste product at the remote locations of oil wells, and what could not be used locally was flared off. However, high pressure gas pipelines were economical where there were large quantities of gas and concentrated markets for that gas.

After the First World War, the great natural gas fields of southwestern Kansas, the Texas and Oklahoma panhandles and New Mexico were just beginning to be developed. Fort Worth, however, already had natural gas, and that coming from the Petrolia field was rich in helium. This gas served to develop the methods for separation of helium devised by the Bureau of Mines. There are two general ways to do this. One is to adsorb all but the helium on cooled activated charcoal, and the other is to liquefy the gas and separate the helium by fractional distillation. The second method proved the most economical, but the first was used to some extent to purify the gas obtained when airships were refilled. Just as in the case of hydrogen, air diffused into the helium and had to be removed to preserve the lifting power, but, fortunately, not to avoid an explosive mixture.

Helium at room temperature is quite an ordinary gas, but its small atomic weight means that it diffuses very rapidly, even through very small apertures. Pumps made for other gases proved very inefficient with helium, until special valve designs were devised. Hydrogen, of course, behaves the same way, but diffuses even more rapidly. Even barrage balloons are now obsolete, so that lighter-than-air craft are no longer of military interest. However, the other military uses of helium have become, if anything, of even greater importance.

Eventually there were nine helium conservation plants in Kansas and the panhandles, connected by a helium pipline from Bushton, in central Kansas, to the Cliffside field just north of Amarillo, with several branches. Helium was stored successfully underground in the Cliffside field as the field itself became depleted. Of course, the problem in helium storage is preventing its leakage. The grade A helium now produced is 99.995% pure, a remarkable achievement.

Gas with 8% helium was discovered near Thatcher, Colorado. However, the formation pressure was low, and the carbon dioxide content was 14%. The Rattlesnake field near Shiprock, New Mexico also had 8% helium, and was nonflammable, so the residual gas could simply be vented. A large helium plant was built at Shiprock, in spite of its distance from the railway, and that water had to be obtained at some distance from the San Juan River. Fuel gas had to be furnished from gas wells 35 miles away, that had gas that would burn. The first well, Navajo No. 1, flowed 30 million cubic feet per day at a wellhead pressure of 2900 psi. The plant went into production in 1944. The gas is now exhausted, and the plant dismantled. This plant is a good example of the limited life of gas fields.

The natural gas fields of the midcontinent are now all nearing exhaustion, and with them will go our principal source of helium. So much helium has been produced in the past that no shortage is envisioned by the authorities, but it is sure to come someday. Helium occurs only exceptionally in natural gas, and the U.S. was fortunate in its abundance. However, like crude oil, iron ore and other finite national resources, it is nearing exhaustion. There is no other source of helium, and no substitute for it.

Why the Inert Gases are Inert

The reason why the inert gases are inert can be quite clearly explained. It is not just haughtiness on their part, but a consequence of quantum mechanics. All atoms are just a very small, massive, nucleus of electrical charge +Ze, that has attracted Z electrons with total charge -Ze to make an electrically neutral assembly. The electrons are accommodated in discrete states that occupy definite volumes. If we start with a nucleus of charge Z and build up the atom by adding electrons one by one, the first electron can occupy a state, or orbital, close to the nucleus, designated 1s. The second electron can also occupy this state, provided that its spin in opposite to that of the first electron. The third electron must go into a state outside of this, and so of less energy, to a state designated by 2s, or to one of the three states designated 2p. Eight electrons can be accommodated in this "shell," after which we must go to states of still higher energy. When we have added Z electrons in this way, the atom is neutral and can form part of macroscopic matter, which cannot be electrically charged because of the strong forces that would result if it were not. If we put the electrons in states of the lowest possible energy, we have the ground state of the atom. Any other distribution gives us an excited state of higher energy. An atom in an excited state will emit electromagnetic radiation and fall into the ground state in a short time.

We wish to compare the behavior of hydrogen, Z = 1, with a nucleus consisting of a proton, and helium, Z = 2, with a nucleus consisting of two protons and two neutrons, strongly held together by nuclear forces. The single electron of hydrogen in its lowest state is in the 1s orbital, from which 13.5 eV are required to eject it. The two electrons in helium are in the 1s orbital with opposite spins, which is the configuration (1s)2. The next electron would have to go into an orbital of considerably higher energy (in lithium), so the 1-, or K-shell is said to be "filled."

If we bring two hydrogen atoms near to one another, the electrons can arrange themselves in different ways, or molecular orbitals, around the two protons. If we imagine the molecular orbital as a linear combination of the atomic orbitals, we can form the orbitals 1s + 1s and 1s - 1s, adding in one case and subtracting in the other. When we normalize the result (make its integral over all space equal to 1) we get two orbitals that are approximately those of the combination of the two atoms. These are designated (σg1s) for the sum, and (σu1s) for the difference. The absolute value squared of these wave functions indicate the average electron density. In the first state, it is easy to see that the electron is mainly between the nuclei, but in the second it is beyond them. Calculation shows that when the electron is between the nuclei, the shielding of the repulsion of the two nuclei is complete enough that the energy is lower than that of the two bare nuclei, which then attract rather than repel. Such an orbital is called bonding. In the other state, the nuclei repel strongly, and the orbital is called antibonding. For one 1s state on each atom, we get two molecular orbitals.

When two hydrogen atoms are brought near to one another, there are two electrons to dispose of, and both can be placed in the bonding 1s molecular orbital. The electrons are mainly between the protons, neutralizing their charge exactly, and causing the protons to attract one another. A potential well is produced that is deepest at some finite separation of the protons. The two atoms then hang together quite strongly, and can vibrate and rotate if thermally agitated. A stable, neutral diatomic molecule has been formed, H2. 4.476 eV is required to break it apart, and this is much greater than the normal thermal energy.

If we do this with helium atoms, the first two electrons will fit into the bonding orbital, as in hydrogen. The third and fourth electrons, however, will have to go into the antibonding orbital, and as a result the atoms will not attract one another enough to stick together; their mutual potential energy curve will have no minimum. Therefore, there is no stable He2 molecule in the ground state. However, if we use the 2s orbitals, then we can form a bonding orbital (σg2s) which will be more favorable than the antibonding 1s orbital. If we put the fourth electron in such a state, then there is sufficient attraction to hold the molecule together, and we have He2*(σg1s)2u1s) (σg2s). The * indicates that the molecule is in an excited state. Obviously, there may be other possibilities, but this is one structure that is observed, and whose spectrum has been investigated.

Another possibility is to ionize one of the atoms, so that there are only three electrons, which can be accommodated so that the atoms stick together. He2+, like He2*, exists and has been studied. Since it is electrically charged, it can occurr only in small amounts and cannot make any bulk matter, because of the strong electrical forces. He2+ will attract an electron, become He2*, which in turn will radiate (emit photons), reach the ground state He2, and fall apart. This is the reason helium is an inert gas, and will not combine with other atoms, just as it will not combine with itself, to form a stable diatomic molecule. Some success has been experienced in forming compounds of inert gases with very strong electron acceptors such as fluorine, but any such compounds are very fragile, and the statement that the inert gases form no chemical bonds is a very good approximation. Each inert gas has a completed outer shell of 8 electrons (configuration s2p6), and the jump to the next available orbitals is a large one, so all the inert gases act the same way.

This is yet another example that atoms or elements have no innate properties. Everything is determined by structure. The visible universe is held together by electrical forces, and governed by quantum mechanics. This explanation of matter has been completely successful, and is of universal application.

The ground state of the helium atom has zero spin, as we have seen, because there is only one orbital available, and two electrons can occupy it only if their spins are opposite, so that the angular momentum adds to zero. If the two electrons are in different orbitals, then their spins may be either opposite and give singlet states, or may be parallel and add to 1, giving triplet states, so called because three different states can be formed that have the same energy in the absence of a magnetic field. For a singlet, only one such state is possible. Any spectral line is the result of a transition from one stationary state to another of the atom. The selection rule that singlets may not combine with triplets is very strictly observed in helium. The spectrum, therefore, seems to be separate spectra of singlet helium, called parhelium, and of triplet helium, called orthohelium, as if these were two different gases mixed in the ratio of 1:3. Of course, this is not really the case, but the strength of the selection rule is brought out. One result is that the triplet state of the configuration 1s2s, the lowest excited configuration, cannot combine with the ground state, and so is called metastable. Any helium that winds up in this state will remain there, unless dislodged by a collision or similar disturbance. The observed helium spectrum is mainly due to excitation of one electron, the other remaining in the 1s state.

The helium atom was an excellent test of the new theory of wave mechanics. The earlier orbit theory of Bohr and Sommerfeld failed completely with hydrogen, but the new mechanics gave results more accurate than the current experimental values for the ground state energy of helium. The first ionization potential of helium is 24.580 V, and the second is 54.403 V, so the ground state is 78.983 eV below the alpha particle and two free electrons. Recall that the ionization potential of hydrogen is 13.5 V.

The Inert Gases as Gases

We note that the hydrogen molecule is really rather inert itself. If it encounters another hydrogen molecule, the situation for the electrons is much as in the case of two helium atoms. There is not enough room to accommodate the four electrons between the nuclear charges, so there is very little attraction. The other diatomic atmospheric gases, oxygen and nitrogen, behave the same way. An oxygen atom is a very active thing, and will combine with about any other available atom. However, the diatomic molecule is practically inert. Nitrogen is similar, but diatomic molecules of oxygen and nitrogen can bumble around in the air with nothing happening. It takes a lightning stroke, which can break up a diatomic molecule, to encourage the formation of only a little nitrogen oxides. The atmosphere is quite safe from becoming nitrogen oxides in a grand conflagration. Similarly, oxygen and hydrogen, as diatomic molecules, can coexist peacefully, at least in the dark. The slightest spark that tears apart a diatomic molecule can release a chain reaction that proceeds explosively. The difference between these diatomic molecules and the inert gases is simply that the diatomic molecules can be torn apart with sufficient violence, while the inert gas atoms cannot be.

Actually, two helium atoms do attract one another slightly because of the interaction of their fluctuating electrical fields, that act like rapidly moving dipoles averaging to zero. These are the London dispersion forces, or Van der Waals forces. These forces become stronger as the number of electrons increases, so that Xenon atoms are relatively sticky. The main force between two helium atoms is a strong repulsion at short distances, so that they resemble hard spheres. Helium is a close approximation to the hard-sphere gas studied in kinetic theory, and so is a good example of the theory. At a temperature T, the average kinetic energy per molecule is 3kT/2, and this is the only internal energy of the gas to a good approximation. This means that the molar specific heat at constant volume is Cv = 3R/2, the molar specific heat at constant pressure is Cp = 5R/2, the ratio of specific heats γ = 5/3, and the gas obeys the ideal gas law pV = RT, where V is the molar volume, and R is the molar gas constant. The speed of sound c is given by c2 = γRT/M. Helium obeys all these relations very well.

Actually, the finite volume of the atoms and the slight attraction between them affect the equation of state, the relation between p, V and T. The van der Waals equation of state, (p + a/V2)(V - b) = RT, accounts for these effects through the constants a and b, different for each gas. If p is in atmospheres, V in litre/mol and T in K, then R = 0.08217. For helium, a = 0.03412 and b = 0.02370. These values are remarkably small, less than for any other gas, and show how close helium is to an ideal gas. When we get to xenon, a = 4.194 and b = 0.05105. The larger, more usual, value of a reflects the greater attraction due to dispersion forces, but the value of b shows that the atoms are still small. The atomic radii, in Å of the inert gases are: He, 0.53; Ne, 1.60; A, 1.91; Kr, 1.98; Xe, 2.18. The diameters range from 1 to 4 Å, typical atomic diameters, small compared to most molecules.

The speed of sound in helium at 300K is 1010 m/s, about three times greater than in air at the same temperature, where the speed is 347 m/s. Acoustic resonators filled with helium will resonate at a higher frequency than those same resonators filled with air. The pitch will be about a twelfth higher, an octave and a half. The same is true for hydrogen, where the speed of sound is 1316 m/s, but the pitch is raised even more, close to two octaves. Professor Tyndall used to amaze the children at the annual Royal Institution lectures by breathing in hydrogen, and then speaking with the unexpectedly squeaky voice that resulted. This, of course, is extremely dangerous and Tyndall was lucky. It is much more safely done with helium (now that helium is available cheaply in the required amounts), and is a familiar party trick. Breathing xenon the same way should make anyone a booming bass, but the trick would be a costly one.

Breathing helium (or any other inert gas except radon) is quite safe, so long as enough oxygen is supplied to avoid asphyxiation. In fact, 80% He, 20% O2 is a good breathing mixture for divers and others working under pressure (with the oxygen content properly regulated). It happens that helium is the gas least soluble in water, only 0.97 parts in 100 parts at 0°C. Nitrogen is soluble to the extent of 2.35 parts. When air is breathed under pressure, nitrogen dissolves in the blood. When the pressure is relieved, the nitrogen comes out of solution as bubbles, which create havoc in the circulatory system, causing the "bends" or "caisson disease." Helium dissolves to a much lesser extent, and decompression is quicker. Helium is also used in breathing mixtures for asthmatics, since its rapid rate of diffusion opens up congested lungs more easily than ordinary air does. In anesthesia, helium is also used as a carrier gas.

The mass of helium on the O = 16 scale is 4.00387 amu. The mass of two hydrogen atoms and two neutrons is 4.03426 amu. The difference, 0.03039 amu, corresponds to the binding energy of the alpha particle, or helium nucleus, about 28.3 MeV. This demonstrates the great strength of nuclear forces, as well as the stability of the alpha particle. Helium is produced in large amounts by thermonuclear reactions in the sun. Hydrogen and helium are, by far, the most common elements in the universe, but only small amounts of them appear on the earth.

Xenon and Radon

These are the heaviest of the inert gases, and have some peculiarities. Xenon is, amazingly, an anaesthetic, in spite of the fact that it is inert. This is not completely explained, but is supposed to be associated with the strange behavior of xenon and water. 46 water molecules can hydrogen-bond themselves into a complex molecule with icosahedral symmetry that forms eight "cages" of the right size to accommodate xenon atoms, stabilized by dispersion forces between the xenon and the water. This 8Xe·46H2O is called a clathrate, from the Greek for a cage. Eight methane molecules can take the place of eight xenon atoms, to form an ice-like methane hydrate that can block pipe lines even at temperatures well above 0°C. Possibly the xenon encourages the formation of such clathrates in the brain, and the other smaller cages that are formed can clathrate ions important in brain function. For more information on gas hydrates, see Hydrates.

Radon is the heaviest inert gas, and all its 15 isotopes, ranging from mass 206 to 222, are radioactive. The one with the longest half-life, Rn222, was the one originally called radon, or radium emanation. It is produced by the alpha decay of ordinary radium, Ra226, which has a half-life of 1620 years, and its half-life is 3.825 days. Small amounts of this gas were repeatedly pumped off a radium sample and sealed in small ampoules for radiation therapy. Radon is also a member of the Actinium and Thorium natural radioactive series, and was originally called Actinon and Thoron in those cases. Actinon and Thoron have half-lives of only 3.92 s and 54.5 s. In 1923 it was decided to call all Z = 86 atoms Radon.

Radon was the basis of a typical scam not long ago, when a homeowner could pay money to have his house examined for deadly radon, another one of the modern homeopathic horrors. Any house built on granitic rock will show some radon, but if the house is reasonably ventilated, there will not be enough radon to do any damage, except in a very few cases when a house has been built on a radium dump or something similar. Actinon and thoron are even more minuscule hazards. Thoron sparked the expensive and superfluous cleanup of the Weshbach gas mantle factory site in Camden, NJ. This is not to say that breathing any considerable amounts of radon is not hazardous, for it certainly is. It is all a matter of amount. The scam now seems to be cleaning air ducts.

Liquid Helium

If a liquid and its vapor are sealed in a container and heated, the pressure in the container, called the vapor pressure, increases steadily, while the vapor becomes denser and the liquid less dense. At a certain temperature, the distinction between vapor and liquid disappears, along with the free surface dividing one from the other. This is the critical point, defined by the critical temperature Tc, the critical pressure pc and the critical specific volume Vc. Above the critical temperature, no pressure is sufficient to produce a liquid, so that to liquefy a gas, the temperature must be below the critical temperature. The change from liquid to vapor involves the absorption of latent heat, and such a phase change is called first order.

For water, whose molecules attract one another, the critical temperature is 647.4K, the critical pressure is 218.3 atm (3210 psi), and the critical density is 0.32 g/cc. By contrast, the critical temperature of helium is 5.2K, the critical pressure is 2.26 atm, and the critical density is 0.0693 g/cc. The critical temperature is so low that helium is very well described as a "permanent gas." Hydrogen has a critical temperature of 33K, critical pressure 12.8 atm, and critical density 0.0310 g/cc, showing how effectively inert its molecules are. Oxygen and nitrogen also have low critical temperatures, 155K and 126K.

One way to produce low temperatures and liquefy gases is by expanding the gas through a porous plug, called throttling, from a high pressure to a low one. There is usually a change in temperature, called the Joule-Kelvin Effect. Although this is an irreversible process, it is easy to show that the enthalpy of the gas is not changed. The enthalpy H is the sum of the internal energy of the gas, U, and the product of pressure and volume: H = U + pV. If we apply the First Law to the process, the heat added, Q, which is zero, is Q = U - U' + W, where W is the work done by pistons moving to keep the pressures constant on the two sides, p on the high pressure side, and p' on the low pressure side. Then, we have 0 = U - U' + pV - p'V', or U + pV = U' + p'V', or H = H'. This makes it easy to predict the change in temperature in the process. An important parameter is the slope of the curves H = constant, or μ = (dT/dp)H, called the Joule-Kelvin coefficient.

Thermodynamics shows us how to express μ in terms of the equation of state of the gas: μ = (1/Cp)[T(dv/dT)p - v]. If we use the ideal gas equation of state, we find that μ = 0. This means that we cannot cool or heat an ideal gas in this way. For a real gas, μ is positive, meaning that the gas cools on throttling, inside a curve on a T,p plot called the inversion curve, as shown in the diagram. It is clear that the gas must be cooled below the temperature To before we can cool it in this way. One isenthalpic curve is shown. If the initial state is on the inversion curve, we get the maximum cooling. The state p,T is on the inversion curve, and the final state p',T' is on the H = constant curve through the initial state. There is a cooling from T to T'. To liquefy a gas, we compress it and throttle it, and pass the cooler gas back in a countercurrent heat exchanger to heat the high-pressure gas. The gas becomes continually cooler, and finally it liquefies and can be drawn off.

For air, To is 603K, so we are well within the inversion curve at room temperature. Therefore, it is relatively easy to liquefy air in this way, so this process is the foundation of a considerable industry. Liquid nitrogen separated from the liquid air boils at 77K, and can be used to precool hydrogen below its To of 202K. Then, we are in a region where the hydrogen can be liquefied by the Joule-Kelvin effect. This is more difficult than liquefying air, but can be done, and the liquid hydrogen boils at 20.4K. To for helium is above 25K, so precooling with liquid hydrogen will bring it inside the temperature range where it can be cooled by throttling, in spite of its small nonideality. The result of this rather difficult process is liquid helium.

Another way to cool a gas is to have it do work adiabatically against a piston in an engine, and this has no temperature boundary like the Joule-Kelvin effect. Gas liquefaction using such engines is now common not only for helium, but also for air. These liquefaction plants are small and easily handled, so that every lab can have its own source of liquid air. The Collins helium liquefier, described in Zemansky (see References) makes even liquid helium readily available. Liquid helium is about 500 times more expensive than liquid air. Helium was first liquefied by Kamerlingh Onnes in 1908, using the Joule-Kelvin effect as we have described.

Liquid helium boils at 4.2K, has a density of about 0.125 g/cc, and an index of refraction of 1.024, close to that of a vacuum. The latent heat of vaporization is only 5 cal/g, so it evaporates rapidly and must be carefully insulated to reduce loss. Helium cannot be frozen at atmospheric pressure. Since it is so light, there is considerable zero-point energy, and this unavoidable shuffling keeps it from forming an orderly crystal. Helium can be frozen at higher pressures, however. At 25 atm it freezes into a hexagonal close packed solid near absolute zero. Even hydrogen freezes at 14K. Because helium remains gaseous when all other gases have liquefied, or even frozen, it has been valuable in rocketry, which uses many cryogenic gases as fuels.

A very curious thing happens at around 2.172K. This point is accurately known because the specific heat has a sudden spike at this temperature, resembling a Greek gamma on a plot of specific heat vs. temperature. Therefore, it is called the gamma point. Helium above the lambda point is called Helium I, and the phase below the lambda point is called Helium II. Below the lambda point, the atoms begin to associate in a single quantum state with a volume equal to that of the liquid. The fraction of atoms in the superfluid ground state is about Ns/N = 1 - (T/Tγ)3/2. This curious state is called a Bose-Einstein condensation, and is a single quantum state with a macroscopic extension. The condensation occurs in momentum space, not in configuration space. Each atom in this state is exactly equivalent to any other, and the state is unchanged on the exchange of any two atoms. This many-atom state is possible because the spin of the helium atom is zero, which requires the wave function of multiple atoms to be symmetric on exchange of any two atoms. It is not a simple state, but has many degrees of freedom, and its own peculiar excitations. Liquid helium II can be considered as composed of two interpenetrating fluids, the superfluid and the normal fluid. It will support normal sound waves, but also waves in which the superfluid and normal fluid move in opposite directions, called second sound, where there are no pressure fluctuations, but temperature fluctuations, or fluctuations in the ratio of the densities of superfluid and normal fluid, instead.

P. Kapitza found that the superfluid component had zero viscosity in 1938, while Allen and Misener discovered the fountain effect in the same year. Helium II had been studied since 1908, and many of its mysterious properties had been noticed before that time. Landau and Tisza proposed the two-fluid theory of helium II in 1941, which clarified much of what had been observed. Superfluid helium will flow without pressure drop through a fine capillary that will completely stop the normal fluid of independent atoms. Because it attracts itself so little, it will wet any surface and flow over it by capillary action, creating a thin film less than 100 nm thick. Flow in this film can serve as a siphon to equalize levels in different containers. The helium in a suspended cup will even run over the rim and drip off the bottom. If a container of it is rotated, the superfluid will not rotate. However, it may be penetrated by quantized vortices that give the macroscopic impression of a mass rotation; all the angular momentum is in these vortices. The quantum of circulation (line integral of the velocity around a closed path) is h/m, where m is the mass of the helium atom.

Helium II is actually a many-particle system in which some of the atoms are in the superfluid ground state, while the remainder are in excited states, whose energies are functions of their momenta. The excited states are described as phonons, quantized vibrations. When the wavelength of a phonon approaches the average interatomic spacing, a new kind of excitations called rotons appear. (These are not the quantized vortices mentioned above.) The "normal" fluid is represented by the atoms taking part in these excitations. The two-fluid picture is only a convenient expression of these facts, and should not be taken literally.

Liquid helium can be cooled evaporatively by pumping on it. Above the lambda point, it boils vigorously like an ordinary liquid, but when the lambda point is passed is suddenly becomes still and evaporates only from the free surface. The reason for this is the extraordinarily high heat conductivity of the superfluid that quickly equalizes the temperature and does not allow any local variations that produce bubbles of vapor. This is not normal heat conductivity, but is essentially a mass motion of the superfluid. The superfluid has zero entropy (it is a single state, and S = k ln W with W = 1).

If helium II is locally warmed, some superfluid must change into normal fluid to take up the entropy dQ/T, since the entropy of the superfluid is zero. This causes superfluid to flow into the warmed region, and normal fluid to flow out. This behavior gives rise to the striking fountain effect of Allen and Misener, and is also related to second sound. In general, if a warmer container is connected to a cooler one by a capillary, the superfluid will flow against a pressure gradient from the cooler container to the warmer. This flow and conversion transports heat very effectively, and is responsible for the very large thermal conductivity of helium II.

The phenomenon of superconductivity was discovered by Onnes in 1911, in mercury cooled to 4.2K. In superconductivity, the electrons form a macroscopic quantum state in which the electrical resistance is precisely zero. In this case, pairs of electrons associate with opposite spins, so that these Cooper pairs have zero spin, and can fall into a Bose-Einstein condensation just as the helium atoms do. Superconductivity usually requires the very low temperatures that can only be obtained with liquid helium. Helium atoms have no charge, and so are not superconducting, but the zero viscosity is analogous to the zero resisitivity. These matters have led to theoretical investigations that have clarified the structure of matter to a large degree. Superconductivity and superfluidity depend on the reduction of thermal agitation to a very small value.

Until the discovery in 1972 of the Yttrium-Barium-Copper Oxide ceramic superconductors with a superconducting temperature of 90K, all superconductors had to be cooled with liquid helium, a difficult and expensive process. The high-temperature superconductors, with transition temperatures as high as 125K, can be cooled with liquid nitrogen. However, they have not proved a great technological bonanza so far, and commercial use of superconductors for things like electrical transmission is still just a hope. They may be a solution to a not particularly pressing problem.

Helium-Three

Practically all helium is He4, with two protons and two neutrons very tightly bound. About 1 part per million of atmospheric helium is the stable lighter isotope He3, with only one neutron. There is only a tenth as much in natural gas helium, confirming its radiogenic origin. No nucleus with just two protons is known, but He6, which is beta-active with a half life of 0.82 s and changes into stable Li6, has been found. He5 has only a very transient existence, almost immediately emitting a neutron.

It is impractical to isolate He3 from natural sources. Supplies sufficient for laboratory work are obtained from the beta-decay of tritium, H3, which has a half-life of 12.3 years. If you fill a bottle with tritium, and wait long enough, you end up with a bottle of He3. One might expect tritium to be the more stable, because of the smaller electrostatic energy, but this is not the case. Tritium turns out to be available because it is used in nuclear weapons, and is manufactured in fission reactors by the capture of thermal neutrons by Li6, which immediately splits into an alpha particle and a triton, the nucleus of H3, tritium. It could also be made by neutron capture by H2, deuterium, but the cross section is 2000 times smaller (0.57 mb vs. 950 mb). The nucleus of He3 is called a helion. A triton, incidentally, combines with a proton at high enough temperatures that the electrostatic repulsion can be overcome to form an alpha particle, with the release of 20 MeV.

The only difference between He3 and He4, besides the mass difference, is that the nuclear spin of He3 is 1/2, making it a fermion, instead of zero. This has the striking result that there is no lambda point as might be expected, and so no superfluid, simply because such a state could not have the necessary symmetry. The mass difference makes the boiling point 3.2K instead of 4.2K, because of the larger zero-point energy for the lighter atom. In 1972, a superfluid He3 was finally observed, at a temperature below 3 mK. The He3 atoms associated in pairs with their nuclear spins aligned, to give spin 1, and the resulting bosons could associate into a superfluid.

Uses of Helium and Argon

The uses of helium depend either on its small atomic mass, or on its chemical inertness. Many applications involve both properties. The uses of argon depend on its inertness. Both helium and argon are available in industrial quantities, and are relatively cheap. The rarer inert gases Ne, Kr and Xe are used in very small amounts in electrical discharges, together with He and A. Xe, as we have remarked, is useful as an anaesthetic.

Helium and argon are used in welding to shield the hot metal from the atmosphere, especially in the case of reactive metals. Classic arc welding uses consumable electrodes coated in a flux. In the heat of the arc, the flux evolves protective gases and forms a liquid slag to protect the metal. In gas metal arc welding (GMAW), helium or argon is blown on the weld, protecting it without producing a slag and excluding oxygen or hydrogen. This seems to have been developed in World War II for welding magnesium, but has now been applied much more generally. It is generally considered necessary for good welds in titanium, as well as for aluminium and stainess steel, where it eliminates weld porosity. In gas tungsten arc welding (GTAW), non-consumable tungsten electrodes are used. Helium is often used as an inert atmosphere in growing semiconductor crystals and for similar processes.

The major use of argon is to fill incandescent lamp bulbs. Although the gas cools the filament somewhat, evaporation of tungsten is discouraged and the life of the lamps is significantly extended.

The small atomic mass leads to large thermal velocities, and this implies rapid diffusion and easy heat transfer. Hydrogen leads in these properties, of course, but helium is not far behind. Helium is used for leak detection in vacuum systems. The gas is blown around likely leak sources, and will diffuse much more rapidly than air through even the smallest opening. The helium is easily detected by an ionization gauge or other means, signalling the presence of a leak. The rapid diffusion makes helium a good carrier gas for gas chromatography. Helium is also used as a driver gas in hypersonic wind tunnels. The high thermal conductivity, and its zero neutron capture cross section, make helium a good coolant in gas-cooled nuclear reactors, though its low density works against it.

Helium gained fame as a lifting gas, and we have seen that this spurred the large-scale production and conservation of the gas, in spite of its inherent rarity. We have already remarked that it is nearly as effective as hydrogen in this respect, as 25 is to 27. The net lift of a balloon is the weight of the air displaced, less the weight of the balloon, lifting gas and ballast. All balloons using hydrogen or helium are strongly affected by the rapid diffusion of these gases, which soon leads to a decrease in lift, which is compensated by throwing ballast overboard. Incidentally, the lifting gas is contained in separate bags like the compartments of a ship, so that the compromise of one is not the loss of all. The use of helium in small balloons instead of hydrogen seems to be a frivolous waste of a valuable substance. Of course, the reason it is used is the elimination of the hazards of hydrogen combustion, perhaps more in the gas sources than in the actual balloon. If the predictions of a "hydrogen economy" do indeed come true, we will hear more about the dangers of hydrogen. All the problems of working with helium arise with hydrogen as well, with the added excitement of explosion.

The inert gases are excellent as filling gases for electrical discharges, since they do not react with the electrodes, and tubes containing them have long service lives. Neon and argon are used in glow discharge lamps, where the light comes principally from the negative glow near the cathode. Neon has its typical orange-red color, while argon produces a violet. The electrodes are specially treated to encourage electron emission, so that the striking voltage of the lamps is low. "Neon lights" use the positive column in a long tube as the light source. Helium gives a yellow light, and neon red-orange. Mercury with argon to aid starting gives blue. Colored glass tubing is used to create other colors. The pressure is a few mm Hg, and the voltage drop around 100 V per metre. These lamps are supplied with a high-voltage transformer with large leakage reactance so that the voltage is high for striking, and decreases when current flows. The current may be 25 to 50 mA.

A discharge through a mixture of helium and neon creates a population inversion in the neon that can be used in a laser. The helium is ionized in the discharge, and when the electrons recombine and cascade down in energy, they pile up in the two metastable states 2s3S (19.82 eV) and 2s1 (20.62 eV) of the electron configuration 1s2s. Fast transitions to the ground state are discouraged by the selection rules, in one case forbidding triplet-singlet transitions and in the other S-S transitions. By good luck, the energies of each of these metastable states is about equal to the energies of two upper levels of laser transitions in neon. The transfer of energy is quite likely, populating these states. One state makes a transition of wavelength 632.8 nm, the familiar He-Ne visible red laser line, to a lower state, while the other makes a transition of 1152.3 nm to the same lower state. This lower state is kept empty by allowed transitions of wavelength 594.5 nm to a still lower state that is depopulated by collisions with the walls of the discharge tube.

All of the inert gases have a large energy gap between the ground state and the first excited state, corresponding to a transition in the far ultraviolet. The visible lines in the spectra are due to transitions between higher states. This explains why the inert gases are transparent, since ordinary visible frequencies are too low to excite them when they are in the ground state. The helium spectrum in a discharge tube shows strong lines at 447.1 nm (blue), 501.6 nm (green), 587.56 nm (yellow) and 667.8 nm (red). This spread of lines creates a whitish light. Neon has a cluster of lines in the red and orange, while argon has groups of lines in the blue and in the red.

References

C. W. Seibel, Helium, Child of the Sun (Lawrence, KS: Univ. Press of Kansas, 1968). The story of the Bureau of Mines helium program, from a central participant.

M. W. Zemansky, Heat and Thermodynamics, 4th ed. (New York: McGraw-Hill, 1957). Chapter 16. A good explanation of low-temperature physics and the empirical properties of liquid helium. For the Joule-Kelvin effect, see pp. 213-216 and 280-287.

L. Pauling and E. B. Wilson, Jr., Introduction to Quantum Mechanics (New York: McGraw-Hill, 1935). Chapters VIII (electron spin and helium) and XII (molecular binding). A complete, accessible, classic account of these matters.

H. E. White, Introduction to Atomic Spectra (New York: McGraw-Hill, 1934). pp. 209-213. Energy level diagram of the helium atom. A classic introduction to the vector model of the atom, including state classification and selection rules.

D. R. Tilley and J. Tilley, Superfluidity and Superconductivity, 2nd ed. (Bristol: Adam Hilger Ltd, 1986).


Return to Physics Index

Composed by J. B. Calvert
Created 14 December 2003
Last revised 29 May 2004