Lithium has two electrons in $\mathrm{1s}$, filling the first energy level; and one valence electron in $\mathrm{2s}$. Could two $\ce{Li}$ atoms form a bond to create $\ce{Li2}$; like $\ce{O2}$ or $\ce{C2}$?
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1$\begingroup$ In an article about relativistic quantum chemistry successfully predicting low mercury melting point, there is mentioned that mercury is one of few elements not forming diatomic molecules in gaseous phase, calling it pseudo noble gas. $\endgroup$– PoutnikCommented Apr 25, 2019 at 5:09
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The dilithium dimer exists- but it isn’t particularly common, and we usually only see it in the gas phase.
Essentially, the 2s electrons interact and form a bonding orbital. (s-p mixing is particularly pronounced here, but I presume you don’t know MO Theory.)
Due to the diffuse nature of lithium’s atomic orbitals, the bonding is pretty weak (Wikipedia gives a binding energy of $102\text{ kJ/mol}$, about one fourth that for a hydrogen molecule); and the inter nuclear distance between the 2 lithium atoms is actually quite large.
answered Apr 25, 2019 at 9:08