Helium(He) is a chemical element under the group of noble gases, inert gases, of the periodic table. With atomic number 2 and atomic weight of 4.002602, helium is the second lightest element and the second most abundant element in the universe surpassed only by hydrogen. It is a colorless, odorless, and tasteless gas with very low boiling and melting points. And it becomes solid only under increased pressure.
The name of helium is derived from the Greek word for sun, Helios, because it was first discovered in the gaseous atmosphere surrounding the sun before it was found on the earth. In 1868, Pierre Janssen, a French astronomer, detected a bright yellow line in the spectrum of the sun while studying a total solar eclipse.That same year, the English astronomer Joseph Norman Lockyer observed the same yellow line in the solar spectrum and found that the line did not correspond to the known lines of sodium around the sun. So, he concluded that it was caused by an unknown element in the Sun that was not yet found on Earth and later named the element helium.
In 1895, the Scottish chemist Sir William Ramsay discovered helium on Earth in a uranium-bearing mineral called cleveite. Investigating the gas produced upon heating cleveite, Ramsay found a unique bright-yellow line in its spectrum matched that of the aforementioned line observed in the spectrum of the Sun. Hence, helium was identified. In 1903, Ramsay and Frederick Soddy further determined that helium is a product of the spontaneous disintegration of radioactive substances.
Even though helium constitutes about 23 percent of the mass of the universe, helium occurs in the Earth’s atmosphere only to 0.0005 percent and small amounts occur in radioactive minerals, meteoric iron, and mineral springs. However, great volumes of helium are found as a component (up to 7.6 percent) in natural gases in the United States and smaller supplies have been discovered in Canada and South Africa and in the Sahara Desert.
Helium gas (98.2 percent pure) is isolated from natural gas by liquefying the other components at low temperatures and under high pressures. Helium is used as an inert-gas atmosphere for welding metals such as aluminum; in rocket propulsion (to pressurize fuel tanks, especially those for liquid hydrogen, because only helium is still a gas at liquid-hydrogen temperature); in meteorology (as a lifting gas for instrument-carrying balloons); in cryogenics (as a coolant because liquid helium is the coldest substance); and in high-pressure breathing operations (mixed with oxygen, as in scuba diving and caisson work, especially because of its low solubility in the bloodstream). Meteorites and rocks have been analyzed for helium content as a means of dating.
Helium is an inert gas and does not easily combine with other elements. There are no known compounds that contain helium, although attempts are being made to produce helium diflouride (HeF2).
Sources:
http://education.jlab.org/itselemental/ele002.html
http://search.eb.com/article-9001713
