Helium

The first evidence of helium (He) was observed in 1868 by the French astronomer Jules Janssen, who observed an unknown yellow line in the spectrum of sunlight during a solar eclipse. Later in the same year, the English astronomer Norman Lockyer observed the same yellow line and concluded that it was caused by an element in the sun that was unknown on Earth. Lockyer and the English chemist Edward Frankland named this new element 'helium', from the Greek helios meaning sun. Helium wasn't isolated on Earth until 1895, when William Ramsay treated cleveite, a uranium-containing mineral, with acids. The Swedish chemists Per Teodor Cleve and Abraham Langlet isolated helium independently by the same method and in the same year. Helium is the only element to be discovered extraterrestrially before being found on Earth.

Helium is the first of the noble gases and is chemically inert under normal conditions, although unstable compounds such as HeNe, HgHe₁₀ and WHe₂ have been created using an electric glow discharge. Helium has the lowest melting and boiling points of all the elements, and except under extreme conditions, it is always observed as a monatomic gas.

One of helium's more amusing properties is the effect it has on the voice. Helium is less dense than air, and so sound waves travel faster in helium than in air. As the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, inhaling helium causes the resonant frequencies of the vocal tract to increase in pitch, resulting in a higher voice. The opposite effect can be achieved by inhaling a gas that is more dense than air, for example sulphur hexafluoride. Be careful playing with gases though, as both helium and sulphur hexafluoride are asphyxiants.

The largest use of helium is as a cryogenic fluid for temperatures at or below 4.2 K. This mostly involves cooling the superconducting magnets in MRI scanners. Helium is also used as a lifting gas for balloons and airships, as it is lighter than air and, unlike hydrogen, is non-flammable.