Question

Stability of the species $Li _{2}, Li _{2}^{-}$and $Li _{2}^{+}$ increases in the order of

• A. $Li _{2}< Li _{2}^{-}< Li _{2}^{+}$
• B. $Li _{2}^{-}< Li _{2}< Li _{2}^{+}$
• C. $Li _{2}< Li _{2}^{+}< Li _{2}^{-}$
• D. $Li _{2}^{-}< Li _{2}^{+}< Li _{2}$

Answer 1

Answer: (D)

$Li _{2}^{-}< Li _{2}^{+}< Li _{2}$
Bond order $\propto$ stability of species
$ Li _{2}=\sigma 1 s ^{2}<\sigma 1 s ^{2}<\sigma 2 s ^{2} ; B.O = \frac{4-2}{2}=1 $
$ Li _{2}^{-}=\sigma 1 s ^{2}<\sigma 0 s ^{2}<\sigma 2 s ^{2}<\sigma 2 s ^{1} ;B. 0=\frac{4-3}{2}=0.5 $
$Li _{2}^{+}=\sigma 1 s ^{2}<\sigma 0 s ^{2}<\sigma 2 s ^{1} ;B.O=\frac{3-2}{2}=0.5$
$Li_{2}^{+}$ and $Li_{2}^{-}$ both have same bond order, but $Li_{2}^{-}$ has more number of electron in anti-bonding molecular orbitals (experience more inter electronic repulsion) and hence least stable among three species.