alleryStability of the species $Li_2$, $Li_2^-$ and $Li_2^+$ increases in the order of:
- $Li_2 < Li_2^+ < Li_2^-$
- $Li_2^- < Li_2^+ < Li_2$
- $Li_2 < Li_2^- < Li_2^+$
- $Li_2^- < Li_2 < Li_2^+$
Show answer and solution
Correct answer: B. $Li_2^- < Li_2^+ < Li_2$
Use molecular orbital theory.
For $Li_2$, the valence configuration is $(\sigma 2s)^2$, so bond order $=\frac{2-0}{2}=1$.
For $Li_2^+$, one electron is removed from the bonding orbital, so bond order becomes $\frac{1-0}{2}=0.5$.
For $Li_2^-$, the extra electron enters the antibonding $\sigma^{*}2s$ orbital
so bond order is also $\frac{2-1}{2}=0.5$, but it is less stable than $Li_2^+$ because the added antibonding electron increases repulsion and weakens the bond more effectively.
**Therefore stability increases as $Li_2^- < Li_2^+ < Li_2$.**
