Li2

Li2 is the diatomic molecule composed of two lithium atoms. It exists primarily in the gas phase under high temperature or energetic conditions and is transparent to many wavelengths of light, making it a useful subject of spectroscopic study. In the lab, Li2 can be produced by combining lithium atoms in a sufficiently energetic environment or by laser ablation of lithium metal.

The electronic structure of Li2 is described by a single bond formed from the overlap of the two lithium 2s valence orbitals. Each lithium atom contributes one valence electron, giving a bond order of 1 in the ground state. The ground electronic state of Li2 is X1Σg+. The valence bond picture corresponds well with a weakly bound diatomic, in contrast to stronger covalent bonds found in many nonmetal molecules.

Physically, the Li–Li bond length is about 2.65 to 2.69 angstroms, reflecting a weak interaction between the two atoms. The bond dissociation energy is approximately 100 to 105 kilojoules per mole (roughly 1.0 to 1.1 eV). Li2 has several excited electronic states, and its potential energy curves have been mapped extensively to support spectroscopic and thermochemical analyses. Rotational and vibrational transitions have been observed and used to refine molecular constants.

Isotopologues such as 6Li2 and 7Li2 exist, with slight shifts in spectroscopic features due to differing nuclear masses. Li2 is widely studied in ultracold-atom research, where it is formed into weakly bound molecules near Feshbach resonances and used to explore quantum degenerate gases with mixed lithium isotopes. Safety considerations reflect the reactivity of lithium metal and compounds, though Li2 as a gas-phase molecule is handled under controlled laboratory conditions.