Dilithium, Li2, is a strongly electrophilic, diatomic molecule comprising two lithium atoms covalently bonded together. Li2 is known in the gas phase. It has a bond order of 1, an internuclear separation of 267.3 pm and a bond energy of 101 kJ mol−1 or 1.03eV in each bond. The electron configuration of Li2 may be written as σ2.
3D model (JSmol)
|Molar mass||13.88 g·mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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It has been observed that 1% (by mass) of lithium in the vapor phase is in the form of dilithium.[clarification needed] Molecules containing more than two lithium atoms covalently bonded together do exist, albeit in smaller quantities than dilithium. Clusters of lithium atoms also exist; the most common arrangement is Li6.
Being the lightest stable neutral homonuclear diatomic molecule after H2, and the helium dimer, dilithium is an extremely important model system for studying fundamentals of physics, chemistry, and electronic structure theory. It is the most thoroughly characterized compound in terms of the accuracy and completeness of the empirical potential energy curves of its electronic states. Analytic empirical potential energy curves have been constructed for the X-state, a-state, A-state, c-state, B-state, 2d-state, and l-state, E-state, and the F-state mainly by professors Robert J. Le Roy of University of Waterloo and Nike Dattani of University of Oxford. The most reliable of these potential energy curves are of the Morse/Long-range variety.
Li2 potentials are often used to extract atomic properties. For example, the C3 value for atomic lithium extracted from the A-state potential of Li2 by Le Roy et al. in  is more precise than any previously measured atomic oscillator strength. This lithium oscillator strength is related to the radiative lifetime of atomic lithium and is used as a benchmark for atomic clocks and measurements of fundamental constants.
|Electronic State||Spectroscopic Symbol||Molecular term symbol||Bond length in pm||Dissociation energy in cm−1||# of bound vibrational levels||References|
|Ground||X||11Σg+||267.298 74(19)||8 516.780 0(23)||39|||
|2||a||13Σu+||417.000 6(32)||333.779 5(62)||11|||
|4||A||11Σg+||310.792 88(36)||9 353.179 5 (28)||118|||
|6||B||11Πu||293.617 142(310)||298 4.444||118|
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