Compounds:

Helium is chemically unreactive under all normal conditions due to its valence of zero. 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 with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe₁₀, WHe₂ and the molecular ions He₂⁺, He₂²⁺, HeH⁺, and HeD⁺ have been created this way. This technique has also allowed the production of the neutral molecule He₂, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000.

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to 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 NMR spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral helium compounds to be formed.