碱金属
Alkali metal helide ions are known for all the alkalis. The molecule ground state for the diatomic ions is in the X1Σ+ state. The bond length gets bigger as the periodic table is descended with lengths of 1.96, 2.41, 2.90, 3.10, and 3.38 Å for Li+He, Na+He, K+He, Rb+He, and Cs+He. The dissociation energies are 1.9, 0.9, 0.5, 0.4 and 0.3 kcal/mol, showing bond energy decreases. When the molecule breaks up the positive charge is never on the helium atom.