|Classification:||Helium is a noble gas and a nonmetal|
|Melting point:||-272.2 oC, 0.95 K|
|Note: At normal atmospheric pressure, helium does not solidify and so has no melting point. The melting point quoted above is under a pressure of 25 atmospheres.|
|Boiling point:||-268.9 oC, 4.2 K|
|Neutrons in most abundant isotope:||2|
|Density @ 20oC:||0.0001787 g/cm3|
|Atomic volume:||27.2 cm3/mol|
|Structure:||usually hexagonal close-packed|
|(v.high pressure needed to solidify helium)|
|Specific heat capacity||5.193 J g-1 K-1|
|Heat of fusion||0.0138 kJ mol-1|
|Heat of atomization||0|
|Heat of vaporization||0.0845 kJ mol-1|
|1st ionization energy||2372.3 kJ mol-1|
|2nd ionization energy||–|
|3rd ionization energy||–|
|Electron affinity||0 kJ mol-1|
|Minimum oxidation number||0|
|Min. common oxidation no.||0|
|Maximum oxidation number||0|
|Max. common oxidation no.||0|
|Electronegativity (Pauling Scale)||–|
|Polarizability volume||0.198 Å3|
|Reaction with air||none|
|Reaction with 15 M HNO3||none|
|Reaction with 6 M HCl||none|
|Reaction with 6 M NaOH||none|
|Atomic radius||31 pm|
|Ionic radius (1+ ion)||–|
|Ionic radius (2+ ion)||–|
|Ionic radius (3+ ion)||–|
|Ionic radius (1- ion)||–|
|Ionic radius (2- ion)||–|
|Ionic radius (3- ion)||–|
|Thermal conductivity||0.15 W m-1 K-1|
|Freezing/Melting point:||-272.2 oC, 0.95 K|
Discovery of Helium
The story of helium’s discovery is interwoven with the discovery of the nature of stars.
At one time people believed we would never know what stars are made of. In 1835 French philosopher Auguste Comte declared, “we shall never be able by any means to study their chemical composition.” (1)
Comte thought we could only learn what star-stuff was if we could get it into the laboratory.
Despite Comte’s pessimism, the method for the discovery of helium and the compositions of the stars had already been found. In 1814 German physicist Joseph Fraunhofer had taken Isaac Newton’s method of splitting sunlight using a prism and had made a crucial advance. Fraunhofer had noticed dark lines in the rainbow of colors coming from sunlight split by a prism; the lines he saw were the first ever observation of a star’s spectrum. (2), (3)
In 1859/60 German scientists Gustav Kirchhoff and Robert Bunsen made enormous leaps in the science of spectroscopy, including the discovery that the dark lines Fraunhofer had seen were like a substance’s fingerprint.
The scene was set for Kirchhoff and Bunsen to discover new elements by studying light from substances when they were burning.
In 1860 they discovered cesium by its blue spectral lines and in 1861 rubidium from two red spectral lines. Then William Crookes discovered thallium in 1861 after observing a bright green spectral line.
Kirchhoff and Bunsen looked at the sun’s spectrum and were able to conclude that iron was present in its glowing atmosphere. (4)
For helium’s discovery, a few more years were needed. In August 1868 the first total eclipse since Kirchhoff and Bunsen’s work had been published was due.
French astronomer Pierre Janssen was waiting for an eclipse in order to observe prominences in the sun’s corona using a spectroscope. In the two weeks following the eclipse Janssen developed a method of recording prominences’ spectra without the need for an eclipse. In these spectra, he observed a yellow line. (5)
The line was in a similar but not identical position to lines in sodium’s spectrum. These were called the D1 and D2 lines. English scientist Norman Lockyer studied the new yellow line; later it would be called the D3 line. He published his study of the line, aware it might be caused by a new element:
“…so then we knew that we were not dealing with hydrogen; hence we had to do with an element which we could not get in our laboratories, and therefore I took upon myself the responsibility of coining the word helium, in the first instance for laboratory use.” (6)
The name helium came from the Greek word for the sun, helios.
Lockyer and Edward Frankland, his coworker, had a number of other ideas about the possible causes of the yellow line and therefore did not announce a new element.
By 1871, other scientists were aware of the situation. Lord Kelvin discussed “reflection of the light of the glowing hydrogen and ‘helium’ round the sun.” The use of ‘helium’ is followed by a footnote to explain it:
“Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium.” (7)
Helium’s existence was not, however, accepted by everyone. (5)
All doubts were dispelled when Scottish chemist William Ramsay isolated helium in 1895 in London. Ramsay had codiscovered argon in 1894; argon was the first of the noble gases to be discovered. In 1895 he read a paper by William Hillebrand describing an unreactive gas that was released when acid was added to the uranium mineral, uranite. Hillebrand believed the gas was nitrogen. [We now know that uranium emits helium during radioactive decay. Radioactivity's existence was not recognized until 1896 when Henri Becquerel's work was published.]
Ramsay, who believed the gas might contain argon, repeated Hillebrand’s experiment using another uranium mineral, cleveite, and collected the gas.
His spectroscope indicated the presence of nitrogen, argon and one other gas. Ramsay suspected it could be helium, because there appeared to be a D3 line. (8) Aware that Lockyer and William Crookes had a better spectroscope than his, he sent them a sample of the gas. Unfortunately the sample was not suitable, so Lockyer obtained a sample of uranite, extracted the gas and studied it by spectroscope. He writes: (8a)
“One by one the unknown lines I had observed in the sun in 1868 were found to belong to the gas.”
The gas’s spectrum was identical to the sun’s ‘helium.’ A new element won its place in the periodic table.
Interesting Facts about Helium
- Helium is the second most abundant element in the universe.
- In 1928 helium became available for the first time on the open market.
- Helium is so light that Earth’s gravity is not strong enough to hold on to it. When helium atoms are released into the atmosphere, they rise until they escape into space.
- Helium is one of only two natural elements that has never been observed bonding to another element in a compound. The other element is neon. Helium plasma can, however, form temporary excimer molecules with elements including sodium, fluorine and sulfur.
- At temperatures close to absolute zero, helium condenses to a liquid with amazing properties – the properties of a superfluid, flowing with zero friction up and over the walls of containers.
- At normal atmospheric pressure, helium does not solidify. At 25 atmospheres of pressure, helium is a solid at 0.95 K. As the pressure rises, the temperature at which solid helium exists also rises. Helium can be made solid at room temperature if the pressure rises to about 114 thousand atmospheres: that is a pressure of 1.67 million psi, or 834 tons per square inch. This is over 100 times greater than the pressure at the oceans’ deepest point, the Challenger Deep, which is almost seven miles deep (10 916 meters).
- Helium exists in Earth’s atmosphere only because it is constantly resupplied from two sources – decay of radioactive elements on Earth, and cosmic rays, about 9% of which are high energy helium nuclei.
- The helium we buy in cylinders is produced by the natural radioactive decay of radioactive elements in the earth’s crust – principally thorium and uranium.
- Radioactive decay of uranium and thorium produces about 3000 metric tons of helium a year.
- Current world production of helium is over 30 000 metric tons a year. (Helium has been accumulating for many millions of years in a few natural gas fields, therefore we can currently extract more each year than is being created by uranium and thorium decay.)
Appearance and Characteristics
Helium is not known to be toxic.
Helium is a light, odorless, colorless, inert, monatomic gas. It can form diatomic molecules, but only weakly and at temperatures close to absolute zero.
Helium has the lowest melting point of any element and its boiling point is close to absolute zero.
Unlike any other element, helium does not solidify but remains a liquid down to absolute zero (0 K) under ordinary pressures.
The voice of someone who has inhaled helium temporarily sounds high-pitched.
Uses of Helium
Magnetic resonance imaging (MRI) is the biggest user of helium. The helium is used to cool MRI scanners’ superconducting magnets.
Helium is used for filling balloons (blimps) and for pressurizing liquid fuel rockets.
Mixtures of helium and oxygen are used as an artificial ‘air’ for divers and others working under pressure. Helium is used instead of the nitrogen in normal air because, after a long dive, helium leaves the body faster than nitrogen, allowing faster decompression.
Helium is used as a gas shield in the vicinity of arc welding preventing, for example, any reaction of hot metal welds with oxygen. The gas is used in the semi-conductor industry to provide an inert atmosphere for growing silicon and germanium crystals. It is also used as a high temperature gas in titanium and zirconium production, and as a carrier gas in gas chromatography.
Abundance and Isotopes
Abundance earth’s crust: 8 parts per billion by weight, 43 parts per billion by moles
Abundance solar system: 23 % by weight, 7.4 % by moles
Cost, pure: $5.2 per 100g
Cost, bulk: $ per 100g
Source: Nearly all the helium on Earth is the result of radioactive decay. The major sources of helium are from natural gas deposits in wells in Texas, Oklahoma and Kansas. Helium is extracted by fractional distillation of the natural gas, which contains up to 7% helium.
Isotopes: Helium has 8 isotopes whose half-lives are known, with mass numbers 3 to 10. Naturally occurring helium is a mixture of its two stable isotopes, 3He and 4He with natural abundances of 0.0001% and 99.999% respectively.
1. Auguste Comte, Cours de Philosophie Positive at Project Gutenberg
2. Keith J. Laidler, The World of Physical Chemistry, 1993, p179.
3. The Era of Classical Spectroscopy, from MIT Spectroscopy.
4. Gerard G. Emch, Mathematical and Conceptual Foundations of 20th-Century Physics., 1984, Elsevier Science Publishers, p211
5. Helge Kragh, The Solar Element: A Reconsideration of Helium’s Early History., 2009, Annals of Science, 66:2, p157-182.
6. J. Norman Lockyer, The Growth of our Knowledge of Helium., Science Progress, 1896, Vol 5., p249
7. W. Thomson, Presidential Address, British Association for the Advancement of Science, 1871, (Address. XCIX).
8. Sir Norman Lockyer, The Sun’s Place in Nature., 1897, p47.
8a. Sir Norman Lockyer, The Sun’s Place in Nature., 1897, p48.
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