You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

Hello, this week we're almost at the top of the periodic table because we're taking a look at the lighter than air gas helium. But for this chemist a helium filled bobbing balloon is actually a source of pain and not a source of pleasure. Here's Peter Wothers.

We are all familiar with the lighter-than-air gas helium, but whenever I see a balloon floating on a string, I feel a little sad. It's not because I'm a miserable old so-and-so - it's just because, unlike the happy child on the other end of the string, I am aware of the valuable resource that's about to be lost forever.

Helium is the second most abundant element in the universe, but here on earth, it's rather rare. Most people guess that we extract helium from the air, but actually we dig it out of the ground. Helium can be found in certain parts of the world, notably in Texas, as a minor component in some sources of natural gas. The interesting thing is how this gas gets into the ground in the first place. Unlike virtually every other atom around us, each atom of helium has been individually formed after the formation of the earth.

The helium is formed during the natural radioactive decay of elements such as uranium and thorium. These heavy elements were formed before the earth but they are not stable and very slowly, they decay. One mode of decay for uranium is to emit an alpha-particle. This alpha-particle is actually just the heart of a helium atom - its nucleus. Once it has grabbed a couple of electrons, a helium atom has been born.

This decay process for uranium is incredibly slow; the time it takes a given quantity of uranium to halve, its so-called half-life, is comparable to the age of the earth. This means that helium has been continuously generated ever since the earth was formed. Some of the gas might eventually creep through the earth and escape into the atmosphere; fortunately, when conditions are right, some is trapped underground and can be harvested for our use.

The situation is very different in space. The sun is comprised of about 75% by mass of hydrogen and 24% of helium. The remaining one percent is made up of all the heavier elements. In the high temperatures of the sun, the hydrogen nuclei are fused together to eventually form helium. This fusion process, whereby heavier atoms are made from lighter ones, liberates vast amounts of energy. Recreating the process on earth may be the answer to our energy problems in the future.

Since helium makes up about a quarter of the mass of the sun, it is not surprising that its presence was detected there over 100 years ago. What is perhaps surprising, is that helium was discovered in space 26 years before it was found on earth.

It has been known for hundreds of years that certain elements impart characteristic colours to a flame - a fact crucial to the coloured fireworks that we enjoy. Copper, for example, gives a green colour, whereas sodium gives a yellow colour. It is actually possible to identify elements by the careful examination of such coloured flames. The light is split up into a spectrum using a prism or diffraction grating in an instrument called a spectroscope. Rather than seeing a continuous rainbow of colours, a series of sharp coloured lines is formed. This series of lines is characteristic of the particular element and acts as a sort of fingerprint.

In the 19th century, scientists turned their spectroscopes to the sun and began to detect certain metals there, including sodium, magnesium, calcium and iron. In 1868 two astronomers, Janssen and Lockyer, independently noticed some very clear lines in the solar spectrum that did not match up to any known metals. While other astronomers of the time were unsure, Lockyer suggested these unidentified lines belonged to a new metal which he named Helium after the Greek personification of the sun, Helios. For over 20 years, no sign of the metal helium was detected on earth and Lockyer began to be mocked for his mythical element. However, in 1895 the chemist William Ramsay detected helium in the gas given out when a radioactive mineral of uranium was treated with acid. The helium formed from the radioactive decay had been trapped in the rock but liberated when the rock was dissolved away in the acid.

Finally Lockyer's element had been discovered on earth, but it was no metal, rather an extremely unreactive gas. To this day, helium remains the only non-metal whose name ends with the suffix -ium, an ending otherwise exclusively reserved for metals.

Aside from being used to fill balloons, both for our entertainment, and for more serious purposes, such as for weather balloons, helium is used in other applications which depend on its unique properties. Being so light, and yet totally chemically inert, helium can be mixed with oxygen in order to make breathing easier. This mixture, known as heliox, can help save new-born babies with breathing problems, or help underwater divers safely reach the depths of the oceans. At minus 269 degrees centigrade, liquid helium has the lowest boiling point of any substance. Because of this, it is used to provide the low temperatures needed for superconducting magnets, such as those used in most MRI scanners in hospitals.

In many facilities where helium is used, it is captured and reused. If it isn't, it escapes into the air. But it doesn't simply accumulate in the atmosphere. Helium is so light that it can escape the pull of the earth's gravitational field and leave our planet forever. This is the fate of the helium in our balloons. Whereas it may be possible to reclaim and recycle other elements that we have used and discarded, when we waste helium, it is lost for good. In 100 years time, people will look back with disbelief that we wasted this precious, unique element by filling up party balloons.

Cambridge University's Peter Wothers telling us the tale of element number two, Helium. Next time we're off to 18^th century Scotland and an element that was the wrong colour.

In 1787, an intriguing mineral came to Edinburgh from a Lead mine in a small village on the shores of Loch Sunart, Argyll. At that time, the stuff was thought to be some sort of Barium compound. Other chemists, such as Edinburgh's Thomas Hope later prepared a number of compounds with the element, noting that it caused the candle's flame to burn red, while Barium compounds gave a green colour.

And that's because it wasn't Barium at all, it was Strontium and Richard Van Noorden will be here to explain how, amongst other things, it's shown us that Roman gladiators weren't meat eaters they were in fact vegetarians. That's next week's Chemistry in its Element and I hope you can join us. I'm Chris Smith, thank you for listening and goodbye.

Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements.