Chapter 9, Problem 51QP

Interpretation Introduction

Interpretation:

The given species ${\text{Li}}_{\text{2}},{\text{Li}}_{\text{2}}{}^{+},{\text{ and Li}}_{\text{2}}{}^{-}$ are to be arranged in increasing order of stability.

Concept introduction:

Two atomic orbitals undergo a combination process in order to form a bonding and an anti-bonding molecular orbital. Orbitals that lie on the internuclear axis combine to form sigma $\sigma$

molecular orbitals, and orbitals parallel to each other combine to form $\pi$

molecular orbitals.

The molecular orbital formed by the combination of $\text{1s}$

orbitals, forms bonding molecular orbital designated as ${\sigma }_{1s}$

and antibonding molecular orbital ${\sigma }^{\ast }{}_{1s}$. The $\text{2s}$

orbital forms corresponding molecular orbitals.

Molecular orbitals formed by the combination of ${\text{2p}}_{\text{x}}$

orbital form a bonding molecular orbital designated as ${\sigma }_{2{\text{p}}_{\text{x}}}$

and an antibonding molecular orbital designated as ${\sigma }^{\ast }{}_{2{\text{p}}_{\text{x}}}$.

Molecular orbitals formed by combining ${\text{2p}}_{\text{y}}$ and ${\text{2p}}_{\text{z}}$

orbital form bonding molecular orbitals designated as ${\text{π}}_{{\text{2p}}_{\text{y}}}$, ${\text{π}}_{{\text{2p}}_{\text{z}}}$ and anti-bonding molecular orbitals designated as ${\text{π}}^{\ast }{}_{{\text{2p}}_{\text{y}}}$

and ${\text{π}}^{\ast }{}_{{\text{2p}}_{\text{z}}}$.

Electrons are filled in the molecular orbitals in the increasing order of energy.

Bond order is determined by subtracting the number of electrons in antibonding orbitals from the number of electrons in bonding and dividing the result by two.

Higher the bond order, more stable is the molecule and bond order zero indicates molecule is not stable.