Question

What does molecular orbital theory explain?

Answer 1

Molecular orbital theory was put forward by Hund and Mulliken, which can be applied to explain the properties, that was not explained by Valence bond theory. This theory explained the paramagnetic nature of $\text{O}_{\text{2}}^{\text{+}}$ion as per Valence bond theory it should be diamagnetic.
Molecular orbital diagram is the diagrammatic representation of all the molecular orbital and electronic configuration of molecular orbitals in a molecule. There are two types of molecular orbitals; bonding molecular orbital and antibonding molecular orbital.
Atomic orbital lose their identity during molecule formation (overlapping) and form a new type of orbitals known as molecular orbitals. The atomic orbitals of comparable energy and proper symmetry combine to form molecular orbitals.

Complete step-by-step answer: Here are the following postulates of molecular orbital theory –
The electron in a molecule is present in the various molecular orbitals. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. The molecular orbital gives electron probability distribution around a group of nuclei in a molecule.
The electron in molecular orbitals is influenced by two or more nuclei depending on the number of atoms in a molecule; hence a molecular orbital is polycentric.
The bonding molecular orbital has lower energy and greater stability in respect to the corresponding anti-bonding molecular orbital.
The order of energies of molecular orbitals for homonuclear diatomic molecule like$\text{O}_{\text{2}}^{{}}$, ${{\text{F}}_{\text{2}}}$and $\text{N}{{\text{e}}_{\text{2}}}$ is-$\text{ }\\!\\!\sigma\\!\\!\text{ 1s}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{1s}$,$\text{ }\\!\\!\sigma\\!\\!\text{ 2s}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{2s}$,$\text{ }\\!\\!\sigma\\!\\!\text{ 2}{{\text{p}}_{z}}$,$\text{ }\\!\\!\pi\\!\\!\text{ 2}{{\text{p}}_{\text{x}}}$= $\text{ }\\!\\!\pi\\!\\!\text{ 2}{{\text{p}}_{\text{y}}}$,${{\text{ }\\!\\!\pi\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{\text{x}}}$=$\,{{\text{ }\\!\\!\pi\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{\text{y}}}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{z}}$.Where- $\text{ }\\!\\!\sigma\\!\\!\text{ }$,$\text{ }\\!\\!\pi\\!\\!\text{ }$ represents the bonding molecular orbital, while${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}$, ${{\text{ }\\!\\!\pi\\!\\!\text{ }}^{*}}$ represents the anti-bonding molecular orbital in a molecular orbital diagram.
The order of energies of molecular orbitals for homonuclear diatomic molecule like${{\text{B}}_{\text{2}}}$, ${{\text{C}}_{\text{2}}}$and ${{\text{N}}_{\text{2}}}$ is-$\text{ }\\!\\!\sigma\\!\\!\text{ 1s}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{1s}$,$\text{ }\\!\\!\sigma\\!\\!\text{ 2s}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{2s}$, $\text{ }\\!\\!\pi\\!\\!\text{ 2}{{\text{p}}_{\text{x}}}$=$\text{ }\\!\\!\pi\\!\\!\text{ 2}{{\text{p}}_{\text{y}}}$ ,$\text{ }\\!\\!\sigma\\!\\!\text{ 2}{{\text{p}}_{z}}$ $\text{,}{{\text{ }\\!\\!\pi\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{\text{x}}}$=$\,{{\text{ }\\!\\!\pi\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{\text{y}}}$,${{\text{ }\\!\\!\sigma\\!\\!\text{ }}^{*}}\text{2}{{\text{p}}_{z}}$.
The stability of molecules can be determined by bond order, higher the bond order higher the stability. The bond order can be calculated as : $\begin{aligned} & \text{Bond}\,\text{order}\,\text{=}\,\dfrac{\text{1}}{\text{2}}\text{(}{{\text{N}}_{\text{b}}}\text{-}{{\text{N}}_{\text{a}}}\text{)} \\\ & \text{ }\\!\\!\\{\\!\\!\text{ }{{\text{N}}_{\text{b}}}\text{=}\,\text{no}\,\text{of}\,\text{bonding}\,\text{electrons,}\,{{\text{N}}_{\text{a}}}\text{=}\,\text{no}\,\text{of}\,\text{antibonding}\,\text{electrons }\\!\\!\\}\\!\\!\text{ } \\\ \end{aligned}$
Higher the bond order, higher is the bond dissociation energy and smaller is the bond length.

Note: Electronic configuration of molecular orbital must be according to Hund’s maximum multiplicity rule, according to which the orbital available in the subshell of a molecule are first filled singly with parallel spin electron before they begin to pair and subshell give maximum number of unpaired electron with parallel spin.