Question

Explain the formation of oxygen molecule.

Answer 1

Molecular Orbital Theory

1. Molecular orbital theory, describes the formation of molecules, by the overlap of two atomic orbitals.
2. It uses Linear Combination of Atomic Orbital ( LCAO) method to represent molecular orbitals.
3. The molecular orbitals are divided into bonding, antibonding and non bonding.

1. Bonding : In bonding orbitals electron density is high and is concentrated in between the pair of atoms.
2. Antibonding : In antibonding orbitals electron density is present behind the nucleus, which tends to pull each of the two nuclei away from the other and actually weakens the bond between the two nuclei.
3. Non bonding - Electrons in non-bonding orbitals are associated with atomic orbitals that do not interact with one another, they do not participate in bond formation.

Formation of Oxygen molecule by Molecular Orbital Theory:

1. Electronic configuration of oxygen $(\mathrm{Z}=8)=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{px}}^{2}2{\mathrm{py}}^{1}2{\mathrm{pz}}^1.$
2. When two oxygen atom combine total electron present in the orbital is 16.
3. MOT combination results in the formation of ${\mathrm{O}}_2:\mathrm{KK}\text{'}{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2<{(\mathrm{\sigma }*2\mathrm{s})}^2<{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2<{\left(\mathrm{\pi }2\mathrm{Px}\right)}^2={\left(\mathrm{\pi }2\mathrm{py}\right)}^2<{(\mathrm{\pi }*2\mathrm{Px})}^1={(\mathrm{\pi }*2\mathrm{py})}^1$ Figure: MOT of oxygen molecule
4. KK' represents the combination of 1s and 2s orbitals(${\left(\mathrm{\sigma }1\mathrm{s}\right)}^2{(\mathrm{\sigma }*1\mathrm{s})}^2$.

Calculation of bond order

On calculating bond order we ignore the combination of inner shells (i.e. KK') as they have two electrons in both bonding and anti bonding orbitals.

Number of electrons in bonding$\left({\mathrm{N}}_{\mathrm{a}}\right)$= 8

Number of electrons in antibonding $\left({\mathrm{N}}_{\mathrm{b}}\right)$=4

$$\begin{gathered} Bondorder=(N_b-N_a)/2 \\ =(8-4)/2 \\ =2 \end{gathered}$$

Hence, the bond order of oxygen molecule is 2.

Oxygen molecule has two bonds. i.e., one is $\sigma$ bond and one $\mathrm{\pi }$ bond. The last 2 electrons in p-orbital acts as lone pair of electrons. The presence of unpaired electron in oxygen atom shows its paramagnetic nature.