Two questions for today.
1) What's the second-most abundant element in the universe?
2) How could we possibly be running out of it on Earth?
Those of you who answered ''helium'' to the first question get full marks. Yes, helium is indeed second only to hydrogen in abundance in the universe, which makes it all the more unusual that there are fears of a helium shortage on Earth at present. The (much longer) answer to the second question may be found below.
Helium is element No 2 on the periodic table, meaning that the nucleus of every helium atom comprises two protons. In addition to these, the majority of helium nuclei contain two neutrons, and there are two electrons associated with each nucleus.
These two electrons are crucial in determining the chemistry of helium as it transpires that this number leads to quite remarkable stability. Helium is a member of a group of elements called the ''noble gases'', so named because they show a marked reluctance to undergo chemical reactions with anything at all.
These gases, which include neon, argon, krypton, xenon and radon, are therefore unique in that they exist in nature as individual atoms rather than bonding to other atoms to form molecules. A
nd this leads to a significant problem in the case of helium atoms, because they are very light. Many of you will have come into contact with helium in the form of party balloons. These are always winners with the children (or children-at-heart) because they float in air. And the reason they do this resides in the make-up of the helium atom discussed above.
Two protons, two neutrons and two electrons give helium gas a mass around that of the same volume of air - therefore, helium is, in the vernacular, lighter than air, and a balloon filled with it will rise. However, if we remove the balloon, then the helium gas will still rise. And this is what is leading to the potential shortage - as helium atoms are so light, they are essentially unaffected by gravity and quite happily float off into space.
Helium is formed by radioactive decay processes in the Earth's crust and, as a result, it is generally obtained by extraction from natural gas deposits. In contrast to its No 2 position in elemental abundance in the universe, it's only No 71 in the Earth's crust on a mass basis, making it about twice as abundant as gold.
And this is a problem, as the major use of helium, funnily enough, is not to fill party balloons or make you talk in a high-pitched voice. It has the distinction of being the lowest boiling element known, only condensing to a liquid at -269degC, and it is used around this temperature to cool the magnets used in MRI (magnetic resonance imaging) instruments in hospitals, and in a variety of other research instruments.
These magnets become superconducting at such temperatures and therefore operate without a continuous input of energy.
Although an enormous amount of research is being carried out into preparing materials that become superconducting at much higher temperatures, at the moment helium is the only viable coolant.
The United States holds the majority of the world reserves of helium in a stockpile, which was slated to close later this year but whose future is now under discussion by Congress.
This has seen the price of helium nearly triple over the past few years, and has led to instrument manufacturers developing helium recycling units to safeguard this very important and dwindling natural resource.
So next time you have a birthday, think about sticking to normal balloons, and leave the helium to the medicos.
- Dr Blackman is an associate professor in the chemistry department at the University of Otago.
