page_title: Helium
section_title: Compounds
hierarchical_section_title: Helium / Compounds
caption_attribution_description: English: Space-filling model of the helium hydride cation, HeH+. Structural data from J. Mol. Model., 2009, 15, 35-40. 日本語: 水素化ヘリウムカチオン HeH+ の空間充填モデル。
context_page_description: Helium is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas, the first in the noble gas group in the periodic table. Its boiling point is the lowest among all the elements. Helium is the second lightest and second most abundant element in the observable universe. It is 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 in both the Sun and in Jupiter. This is due to the very high nuclear binding energy 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, the vast majority of which was 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 Titan of the Sun, Helios. It was first detected as an unknown, yellow spectral line signature in sunlight, during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R.
context_section_description: Helium has a valence of zero and is chemically unreactive under all normal conditions. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to a glow discharge, to electron bombardment, or reduced to plasma by other means. The molecular compounds HeNe, HgHe₁₀, and WHe₂, and the molecular ions He⁺
  ₂, He²⁺
  ₂, HeH⁺
  , and HeD⁺
   have been created this way. HeH⁺ is also stable in its ground state, but is extremely reactive—it is the strongest Brønsted acid known, and therefore can exist only in isolation, as it will protonate any molecule or counteranion it contacts. This technique has also produced the neutral molecule He₂, which has a large number of band systems, and HgHe, which is apparently held together only by polarization forces.
  Van der Waals compounds of helium can also be formed with cryogenic helium gas and atoms of some other substance, such as LiHe and He₂.
  Theoretically, other true compounds may be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000. Calculations show that two new compounds containing a helium-oxygen bond could be stable. Two new molecular species, predicted using theory, CsFHeO and N(CH₃)₄FHeO, are derivatives of a metastable FHeO⁻ anion first theorized in 2005 by a group from Taiwan. If confirmed by experiment, the only remaining element with no known stable compounds would be neon.
  Helium atoms have been inserted into the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable at high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.
  Under high pressures helium can form compounds with various other elements. Helium-nitrogen clathrate (He(N₂)₁₁) crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell. The insulating electride Na₂He has been shown to be thermodynamically stable at pressures above 113 GPa. It has a fluorite structure.
caption: Structure of the helium hydride ion, HHe+
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