| 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+, He2++, 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, like helium fluorohydride (HHeF), may also be possible.
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