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Sodium not only forms species with two electrons in the $3s$ subshell ($\ce{Na^-,Na2}$). It is also known to form a dimer of the $\ce{Na^-}$ion, even though the individual atoms therein possess closed-shell electronic structures and would not he expected to form a covalent bonds to offset the repulsion between the negative charges. The dimeric \$\ce{Na_2^{2-}}...