Question

What is the bond order of Li2−? Is Li2− paramagnetic or diamagnetic?

Answer 1

Concepts and reason

According to molecule orbital theory, bond order of a diatomic molecule or ion depends on the number of electrons in bonding molecular orbitals and antibonding molecular orbitals.

A diatomic molecule or ion with unpaired electrons in its molecular orbitals is paramagnetic. A diatomic molecule or ion with paired electrons in its molecular orbital is diamagnetic.

Fundamentals

Electrons enter first in a molecular orbital with lowest energy. The order of increasing energy of molecular orbitals is $\sigma 1s < {\sigma ^*}1s < \sigma 2s < {\sigma ^*}2s$ .

Bonding molecular orbitals are $\sigma 1s,\sigma 2s$ and antibonding molecular orbitals are ${\sigma ^*}1s,{\sigma ^*}2s$ .

Formula to calculate bond order is ‘ ${\rm{bond order = }}\left( {{N_{\rm{b}}} - {N_{\rm{a}}}} \right)/2$ ’. Here, ${N_{\rm{b}}}$ are number of electrons in bonding orbitals and ${N_{\rm{a}}}$ are number of electrons in antibonding orbitals.

Total number of electrons in ${\rm{L}}{{\rm{i}}_{\rm{2}}}^{1 - }$ are 7.

Molecular orbital electron configuration of ${\rm{L}}{{\rm{i}}_{\rm{2}}}^{1 - }$ is as follows:

$\sigma 1{s^2}{\sigma ^*}1{s^2}\sigma 2{s^2}{\sigma ^*}2{s^1}$

$\begin{array}{c}\\{\rm{Bond order = }}\frac{{{N_{\rm{b}}} - {N_{\rm{a}}}}}{2}\\\\{\rm{ = }}\frac{{\left( {2 + 2} \right) - \left( {2 + 1} \right)}}{2}\\\\ = \frac{{4 - 3}}{2}\\\\ = 0.5\\\end{array}$

The molecular ion ${\rm{L}}{{\rm{i}}_{\rm{2}}}^{1 - }$ is paramagnetic.

Ans:

Bond order of ${\rm{L}}{{\rm{i}}_{\rm{2}}}^{1 - }$ is 0.5

The molecular ion ${\rm{L}}{{\rm{i}}_{\rm{2}}}^{1 - }$ is paramagnetic.