Stability of the species ${\text{Li}}_2$, ${\text{Li}}_2^{-}$ and ${\text{Li}}_2^{+}$ increases in the order of:

We use Molecular Orbital (MO) Theory to determine the Bond Order (B.O.):${\text{Li}}_2$ (6 electrons): $\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2$. $\text{B.O.}=\frac{4-2}{2}=1.0$${\text{Li}}_2^{+}$ (5 electrons): $\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^1$. $\text{B.O.}=\frac{3-2}{2}=0.5$${\text{Li}}_2^{-}$ (7 electrons): $\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^1$. $\text{B.O.}=\frac{4-3}{2}=0.5$ Comparing stability:Higher bond order means higher stability, so ${\text{Li}}_2$ is the most stable.Between ${\text{Li}}_2^{+}$ and ${\text{Li}}_2^{-}$, both have B.O. 0.5, but ${\text{Li}}_2^{-}$ has an electron in an antibonding orbital (${\sigma }^{\ast }2s$), which increases electronic repulsion and decreases stability. Thus, ${\text{Li}}_2^{+}>{\text{Li}}_2^{-}$.Stability order: ${\text{Li}}_2^{-}<{\text{Li}}_2^{+}<{\text{Li}}_2$.