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: (B)

$Li_{2}:\sigma _{1 s}^{2} \, \sigma _{1 s}^{* 2} \, \sigma _{2 s}^{2}$ Bond order $=1$
$Li_{2}^{+}:\sigma _{1 s}^{2} \, \sigma _{1 s}^{* 2} \, \sigma _{2 s}^{1}$ Bond order $=0.5$
$Li_{2}^{-}:\sigma _{1 s}^{2} \, \sigma _{1 s}^{* 2} \, \sigma _{2 s}^{2} \, \sigma _{2 s}^{* 1}$ Bond order $=0.5$
$Li_{2}^{-}$ is less stable than $Li_{2}^{+}$ because the incoming electron goes to anti-bonding molecular orbital. So, the stability order is $Li_{2}>Li_{2}^{+}>Li_{2}^{-}$ .