Chemical Bonding Result Question 7
7. According to molecular orbital theory, which of the following is true with respect to $\mathrm{Li}_2^{+}$ and $\mathrm{Li}_2^{-}$ ?
(2019 Main, 9Jan I)
(a) Both are unstable
(b) $\mathrm{Li}_2^{+}$ is unstable and $\mathrm{Li}_2^{-}$ is stable
(c) Both are stable
(d) $\mathrm{Li}_2^{+}$ is stable and $\mathrm{Li}_2^{-}$ is unstable
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Answer:
Correct Answer: 7. ( d )
Solution:
- Considering molecular orbital theory (MOT) :
The electronic configuration of $\mathrm{Li}_2^{+}(Z=5)=\sigma l s^2, \sigma^* 1 s^2, \sigma 2 s^1$
Bond order $(\mathrm{BO})=\frac{N_b-N_a}{2}=\frac{3-2}{2}=\frac{1}{2}$
The electronic configuration of $\mathrm{Li}_2^{-}(Z=7)=\sigma 1 s^2, \sigma^* 1 s^2, \sigma 2 s^2, \sigma^* 2 s^1$
Bond order $(\mathrm{BO})=\frac{N_b-N_a}{2}=\frac{4-3}{2}=\frac{1}{2}$
For the species having the same value of $BO$ , the species having lesser number of antibonding electrons $\left[N_a\right]$ will be more stable.
Here, $N_a$ of $\mathrm{Li}_2^{+}(2)$ $<N_a$ of $\mathrm{Li}_2^{-}(3)$.
So, their order of stability will be $\mathrm{Li}_2^{+}>\mathrm{Li}_2^{-}$.
BETA
