What is Helium ?

In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed because helium does not solidify at atmospheric pressure.

Onnes' student Willem Hendrik Keesom was eventually able to solidify 1 cm3 of helium in 1926 by applying additional external pressure.

The related stability of the helium-4 nucleus and electron shell

The nucleus of the helium-4 atom is identical with an alpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud.

This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1st orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin.

Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable)....!

This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature....!

For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements......!

Helium shows up in semiconductors, birthday balloons and the Large Hadron Collider. Because of its extremely low density, helium floats in air.

Helium is the second least reactive noble gas after neon, and thus the second least reactive of all elements....

It is chemically inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen.

The low density is also responsible for the weird "squeaky voice" effect when helium is inhaled.

For these reasons and the small size of helium monatomic molecules, helium diffuses through solids at a rate three times that of air and around 65% that of hydrogen...

The Federal Helium Reserve, operated by the U.S. Bureau of Land Management, is near Amarillo, Texas. It holds over 40 percent of the helium used annually in the United States.

Helium is a noble gas, a group of nonreactive and stable gases that also include neon, argon, krypton, xenon and radon.
As of 2010, it took 300,000 cubic feet of helium to float the 15 character balloons in that year's Macy's Thanksgiving Day Parade, the equivalent volume of 2.2 million gallons of milk.

Helium is the least water-soluble monatomic gas, and one of the least water-soluble of any gas (CF4, SF6, and C4F8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5), and helium's index of refraction is closer to unity than that of any other gas.

Helium has a negative Joule–Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule–Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion.

Once precooled below this temperature, helium can be liquefied through expansion cooling...!

Liquid helium

Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) of pressure....

Helium on Earth is formed when radioactive elements in the crust decay, producing positively charged particles called alpha particles. When these alpha particles attract a couple of electrons, the result is a helium atom.

It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%. With a bulk modulus of about 27 MPa it is ~100 times more compressible than water.

Solid helium has a density of 0.214±0.006 g/cm3 at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is 0.187±0.009 g/cm3.[85] At higher temperatures, helium will solidify with sufficient pressure. At room temperature, this requires about 114,000 atm.....

Helium is helping scientists peer past the limits of their knowledge about physics and chemistry, thanks to its convenient ability to become a superfluid with relative ease.

In 1868, Pierre J. C. Janssen travelled to India to measure the solar spectrum during a total eclipse and observed a new yellow line which indicated a new element. Joseph Norman Lockyer recorded the same line by observing the sun through London smog and, assuming the new element to be a metal, he named it helium.

In 1882, the Italian Luigi Palmieri found the same line the spectrum of gases emitted by Vesuvius, as did the American William Hillebrand in 1889 when he collected the gas given off by the mineral uraninite (UO2) as it dissolves in acid. However, it was Per Teodor Cleve and Nils Abraham Langer at Uppsala, Sweden, in 1895, who repeated that experiment and confirmed it was helium and measured its atomic weight.

Helium is the 2nd most abundant element in the universe with the symbol of He. Helium is an Noble gas with 2 protons and 2 electrons with 2 neutrons.