Helium
Symbol: He
Atomic Number: 2
Atomic mass: 4.002602 amu (atomic mass units)
Melting point: minus 272.0 C (1.15 K or minus 457.6 F)
Boiling point: minus 268.6 C (4.548884 K or minus 451.48 F)
Number of Protons: 2
Number of Electrons: 2
Number of Neutrons: 2
Classification: Noble Gas
Crystal structure: Hexagonal
Density @ 283 K: 0.0001785 grams per centimeter
Color: colorless
Helium was discovered in the sun before it was found on Earth. During the eclipse of the sun in 1868 the astronomer Pierre-Jules-Cesar Janssen was studying the sun’s spectrum and found a yellow line at 587.49 nanometers. It was the English astronomer Sir Norman Lockyer who realized that there was no element currently known on earth that could produce such a line. The new element was named helium after the Greek sun god “Helios” by Lockyer.
In 1895 the Scottish chemist Sir William Ramsay was conducting experiments on the uranium containing mineral ore cleveite. After treating the ore with acids he collected the gases that had been produced. Ramsay sent samples of the gases to Lockyer and Sir William Crookes who were able to confirm that helium had been produced on earth. At the same time as Ramsay had produced helium two Swedish chemists, Nils Langlet and Per Theodor Cleve, also working on cleveite did the same.
Helium is an inert gas with no known compounds although attempts are being made to produce helium difluoride. It is constantly being loss from the atmosphere as the molecules escape into space. The element is the second most abundant in the universe after hydrogen but earths’ atmosphere contains only 0.0005% helium. The level in the atmosphere is replenished as it produced by radioactive decay of other elements. The alpha particle produced by radioactive decay is a double helium ion; it can collect two electrons from other elements to become a helium atom.
There are two stable isotopes of helium by far the most common is helium-4 which accounts for 99.999863% of the total abundance. Six unstable isotopes have been found all with very short half lives the longest being 806.7 milliseconds belonging to helium-6.
Helium is obtained commercially from natural gas deposits. It has a numbers of uses.
* Helium is used to fill blimps, party balloons and scientific balloons as it is a safer alternative than the lighter gas hydrogen.
* It provides an inert atmospheric shield for arc welding.
* It is used in supersonic wind tunnels.
* It is used to pressurize the fuel tanks used by liquid fueled rockets.
* Deep sea divers use it in tanks with oxygen to avoid the dangers of nitrogen narcosis. The ratio used is 80% helium to 20% oxygen.
* Helium can provide a protective atmosphere for the growing of silicon and germanium crystals.
* Liquid helium is used as a cryogenic medium particularly in the study of superconductivity.
* It acts as a cooling agent for nuclear reactors.
