# The periodic table/Helium
## Discovery

Helium was discovered in 1895 by Sir William Ramsey in London, UK, and independently by Per Theodor Cleve and Nils Langlet in Uppsala, Sweden. The name is derived from the Greek, 'helios' meaning sun, as it was in the sun's corona that helium was first detected.
Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum while studying a total solar eclipse in 1868. Sir Norman Lockyer, an English astronomer, knew it could not be produced by any element known at the time. It was hypothesized that a new element on the sun was responsible for this yellow emission. In 1895, Sir William Ramsay, a Scottish chemist, conducted an experiment with a mineral called clevite. He exposed the clevite to mineral acids and collected the gases that were produced. He then sent a sample of these gases to two scientists, Lockyer and Sir William Crookes, who were able to identify the helium within it. Two Swedish chemists, Nils Langlet and Per Theodor Cleve, independently found helium in clevite at about the same time as Ramsay.

## Quick Facts

| Name: Helium Symbol: He Mass: 4.002602 Atomic Number: 2 Electron Configuration: 1s$^2$ | Classification: noble gas CAS Number: 7440-59-7 Appearance: colourless, odourless gas Discovery in: 1895 Key Isotopes: $^4$He | Density: 0.1785 g/L Crystal Structure: hexagonal Melting Point: -272.2 °C Boiling Point: -268.93 °C |

## Uses

Helium is widely used as an inert gas shield for arc welding, and as a protective gas for growing silicon and germanium crystals as well as for titanium and zirconium production. It is used as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. A mixture of 80% helium and 20% oxygen is used as an artificial atmosphere for divers and others working under pressure. Helium is extensively used for filling balloons as it is a much safer gas than hydrogen. One of the recent largest uses for helium has been for pressurizing liquid fuel rockets.
Helium has no known biological functions. However, it is non-toxic.
Helium is the second most abundant element in the universe. It is present in all stars. It was and still is being formed from alpha particle decay of radioactive elements in the Earth. Some of the helium formed seeps up to the Earth’s atmosphere which contains about 5 parts per million by volume, but this is a dynamic balance, as the light helium atoms continually escape to outer space.

## Atomic Data

| Atomic radius: 1.400 Å Covalent radius: 0.37 Å Electronegativity: Unknown Electron affinity: Unstable | Ionisation energies First: 2372.323 kJ mol$^{-1}$ Second: 5250.512 kJ mol$^{-1}$ |

## Supply Risk

Scarcity factor: 6.5 (high risk)
| Country with largest reserve base: USA Reserve base distribution: 40.8% Production concentration: 80.3% | Top 3 countries for mining: USA Qatar Algeria | Top 3 countries for production: USA Algeria Russia |

## Oxidation States and Isotopes

Common oxidation states: Unknown
Isotopes
| Isotope | Atomic mass | Abundance (%) | Half life | Mode of decay |
| $^3$He | 3.016 | 0 | | |
| $^4$He | 4.003 | 100 | | |

## Pressure and Temperature Data

Molar heat capacity: 20.786 J mol$^{-1}$ K$^{-1}$