Helium From Wikipedia, the free encyclopedia This article is about the chemical element. For other uses, see Helium (disambiguation). Page semi-protected - ↑ He ↓ Ne hydrogen ← helium → lithium Helium in the periodic table Appearance colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field Spectral lines of helium General properties Name, symbol, number helium, He, 2 Pronunciation /ˈhiːliəm/ hee-lee-əm Element category noble gases Group, period, block 18 (noble gases), 1, s Standard atomic weight 4.002602(2) Electron configuration 1s2 2 Physical properties Phase gas Density (0 °C, 101.325 kPa) 0.1786 g/L Liquid density at m.p. 0.145 g·cm−3 Liquid density at b.p. 0.125 g·cm−3 Melting point (at 2.5 MPa) 0.95 K, −272.20 °C, −457.96 °F Boiling point 4.222 K, −268.928 °C, −452.070 °F Triple point 2.177 K, 5.043 kPa Critical point 5.1953 K, 0.22746 MPa Heat of fusion 0.0138 kJ·mol−1 Heat of vaporization 0.0829 kJ·mol−1 Molar heat capacity 5R/2 = 20.786 J·mol−1·K−1 Vapor pressure (defined by ITS-90) P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1.23 1.67 2.48 4.21 Atomic properties Oxidation states 0 Electronegativity no data (Pauling scale) Ionization energies 1st: 2372.3 kJ·mol−1 2nd: 5250.5 kJ·mol−1 Covalent radius 28 pm Van der Waals radius 140 pm Miscellanea Crystal structure hexagonal close-packed Helium has a hexagonal close-packed crystal structure Magnetic ordering diamagnetic[1] Thermal conductivity 0.1513 W·m−1·K−1 Speed of sound 972 m·s−1 CAS registry number 7440-59-7 History Naming after Helios, Greek god of the Sun Discovery Pierre Janssen, Norman Lockyer (1868) First isolation William Ramsay, Per Teodor Cleve, Abraham Langlet (1895) Most stable isotopes Main article: Isotopes of helium iso NA half-life DM DE (MeV) DP 3He 0.000137%* 3He is stable with 1 neutron 4He 99.999863%* 4He is stable with 2 neutrons *Atmospheric value, abundance may differ elsewhere. v t e · references Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Helium is the second lightest element and is the second most abundant element in the observable universe, being present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this figure in the Sun and in Jupiter. This is due to the very high nuclear binding energy (per nucleon) of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios. It was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer. Jannsen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, which he named. The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas today. Helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships.[2] As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II) is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is relatively rare — 5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Helium is a finite resource and is one of the few elements with escape velocity, meaning that once released into the atmosphere, it escapes into space.[3][4][5] Contents [hide] 1 History 1.1 Scientific discoveries 1.2 Extraction and use 2 Characteristics 2.1 The helium atom 2.1.1 Helium in quantum mechanics 2.1.2 The related stability of the helium-4 nucleus and electron shell 2.2 Gas and plasma phases 2.3 Solid, liquid, and superfluid phases 2.3.1 Helium I state 2.3.2 Helium II state 3 Isotopes 4 Compounds 5 Occurrence and production 5.1 Natural abundance 5.2 Modern extraction and distribution 5.3 Conservation advocates 6 Applications 6.1 Controlled atmospheres 6.2 Gas tungsten arc welding 6.3 Minor uses 6.3.1 Industrial leak detection 6.3.2 Flight 6.3.3 Minor commercial and recreational uses 6.3.4 Scientific uses 7 Inhalation and safety 7.1 Effects 7.2 Hazards 8 See also 9 References 10 Bibliography 11 External links History Scientific discoveries The first evidence of helium was observed on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India.[6][7] This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 Fraunhofer line because it was near the known D1 and D2 lines of sodium.[8] He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek