Helium compounds

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Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly assumed that helium compounds could not exist at all, or at least not under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small, with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom; however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point (4.2 K) of any known substance. Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen. Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionized helium (He+), also known as He II, is a very high energy material able to extract an electron from any other atom. He+ has an electron configuration like hydrogen, so as well as being ionic, it can form covalent bonds. Excimers do not last for...