We continue our review of some of the most important chemical elements in heat treatment and metallurgy.
Helium (Chemical symbol: He)
Helium is an inert gas and does not easily combine with other elements. There are no known compounds that contain helium, and it has never been observed to bond with another element. It is the second most abundant element in the universe, after hydrogen.
Helium was first detected on the sun when Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum during a solar eclipse in 1868. Sir Norman Lockyer, an English astronomer, realized that this line, with a wavelength of 587.49 nanometers, could not be produced by any known element and must be new. This unknown element was named helium by Lockyer after the Greek god of the sun, Helios. In 1895, helium was discovered on Earth when Sir William Ramsay, a Scottish chemist, conducted an experiment with clevite, an impure radioactive variety of uraninite mineral. He exposed the clevite to acids and collected the gases that were produced. Upon analysis, one of them was discovered to be helium.
The earth generates helium during the decay of radioactive elements in the earth's crust, where it seeps to the surface through cracks. It is too light to be gravitationally bound to Earth, and it escapes into space. Helium is commercially recovered from natural gas deposits.
Helium is very important in industrial, home and many other uses, primarily because of its inert properties and low density (lightness). Its many uses include the following:
- To fill airships, party balloons and parade balloons due to its lightness and nonflammable nature.
- To purge fuel and oxidizer from space-support vehicles and to cool hydrogen fuel in space vehicles. To condense hydrogen and oxygen to make rocket fuel.
- Cryogenics, due to its low boiling point and low density. It is mainly used to cool superconducting magnets in MRI scanners.
- To pressurize and purge industrial equipment of unwanted gases.
- As a protective gas when growing silicon and germanium crystals and when producing titanium and zirconium.
- As a heat-transfer medium in certain gas-cooled nuclear reactors.
- As a shielding gas in the arc-welding process, especially with aluminum and copper.
- To detect leaks in high-vacuum and high-pressure equipment. Helium is used to detect leaks (small fractures) in some vessels since it diffuses through solids much faster than air.
- As a carrier gas in gas chromatography.
- In solar telescopes to reduce the distortion due to temperature variations between the lenses.
- In supersonic wind tunnels because of its inertness, high speed of sound and high heat capacity.
- Added to oxygen tanks to help divers breathe more easily, especially for diving over 450 feet below sea level, where the helium-oxygen mix reduces the risk of narcosis and high-pressure nervous syndrome.
It is also important to note that helium is not a renewable resource (i.e., it cannot be manufactured). Once it is released into Earth’s atmosphere, it will rise and escape into space. Here are a few important facts about helium.
- Atomic number: 2
- Atomic weight: 4.002602
- Melting point: 0.95 K (-272.2°C or -458.0°F)
- Boiling point: 4.22 K (-268.93°C or -452.07°F)
- Density: 0.0001785 grams per cubic centimeter
- Phase at room temperature: Gas
- Element classification: Nonmetal