Helium is the second most abundant element in the universe and is primarily produced because of radioactive decay. It was discovered in 1868 by Jules Janssen and Norman Lockyer.
History and Discovery
The discovery of helium is linked with its primary sources, the sun. In the solar eclipse of 1868, several scientists studied the spectral lines coming from the sun and observed the presence of distinct and unknown yellow lines. These lines were named “Helium” by Norman Lockyer, from the word Helios, which is the name of Greek God of the Sun . Helium was isolated by William Ramsay in 1895. Helium was used as lifting gas for air craft in World War I and in World War II, helium was produced widely and used in welding of shielded arc and lifting gas.
|Periodic Table Classification||Group 18
|State at 20C||Gas|
|Density||0.18 g.cm-3 at 20°C|
|Atomic Mass||4.00 g.mol -1|
|Electronegativity according to Pauling||N/A|
Helium is present rarely on the Earth. However, it is the 2nd most abundant element in the universe . The atmospheric content of helium on the Earth is only 5.2 ppm . There is a continuous production of helium on the Earth (via radioactive decay), but it readily escapes the Earth’s atmosphere and enter the space. Helium is the most abundant gas in the Earth’s heterosphere (the layer of atmosphere around 80km above Earth). The primary source of helium on Earth are minerals of thorium and uranium, which emit alpha particles (helium nuclei). About 3000 metric tons of helium are produced annually in the lithosphere (the layer of Earth including upper mantle and the crust) . USA has been the biggest producer of helium since its discovery, but till 2012, most of the helium reserves have been exhausted and now account for 30% of the world’s helium supply. New reserves of helium have been discovered in North America. Russia and Qatar have also developed helium production plants.
Helium is a colorless and odorless gas. It has the lowest melting point among all elements. Its boiling point is close to absolute zero. Helium is hardly soluble in water and is in fact, the least soluble monoatomic gas. Helium is present in plasma state on Earth. The atomic state of helium is predominant in the outer earth and space. There is a considerable difference between the two states of helium. The plasma state of helium has high electrical conductivity. Helium is also highly affected by the magnetic field and interacts with Earth’s magnetosphere to from aurora. Helium remains in liquid form at absolute zero, at standard pressure. This occurs due to quantum mechanics as the zero-point energy of helium is significantly high and does not allow freezing. Helium solidifies at -272C at a pressure of 2.5 MPa (25 bar). The refractive index of solid and liquid helium is almost the same and it is difficult to distinguish between the two states. Helium can solidify at higher temperature under high pressure.
Helium is an inert (Nobel) gas under all standard conditions . In plasma form or subjected to electron bombardment, helium can from unstable compounds with certain metals including, sulfur, iodine, phosphorus and tungsten. These compounds are termed as excimers. Helium can exist in molecular ion form, such as HeH+, which is a highly stable but reactive form of helium. Other compounds, known as Van der Waals compounds of helium are also formed with lithium and cryogenic helium gas. Under high pressure, however, helium can form various compounds, for example, helium-nitrogen clathrate (He(N2)11).
Significance and Uses
- Helium is widely used as coolant in magnetic resonance imaging (MRI) for medical scanning.
- It is used in supersonic wind tunnels.
- Helium is used in arc welding processes.
- Helium is used as an ideal gas for filling of balloons, and airships, as it is lighter than air and is non-flammable.
- Helium is used to detect cracks in buildings with high-vacuum rooms, and high-pressure containers, as it can efficiently diffuse through solids.
- Helium is used as carrier gas in chromatography technique.
Helium is non-toxic but can lead to poisonous effects if inhaled in high concentrations, it can lead to suffocation and death. Minor inhalation of helium can lead to change in the voice (becomes reedy) of individual, as sound waves travel faster in helium as compared to air. This effect is short term and not dangerous .
Isotopes of Helium
There are nine isotopes of helium, and only two are stable, helium-3 and helium-4. The two stable isotopes occur in a ratio of 1:1000,000 in the Earth’s atmosphere. The most abundant isotope (helium-4) is produced because of alpha decay of radioactive elements, such as uranium. Helium-3 is present in scarce amount on Earth and is formed by beta decay of tritium. Helum-4 is highly stable isotope. It is also formed as a result of Big Bang nucleosynthesis. Helim-3 is found in abundance in stars. It is also present on the surface of the Moon surface. Artificial isotopes of helium are also present, including helium-6, helium- and helium-8, where helium-7 and helium-8 are produced during various nuclear reactions .
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