Question

Explain the formation of nitrogen molecule by molecular orbital theory (MOT) .

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, hence they do not participate in bond formation.

Formation of Nitrogen molecule by Molecular Orbital Theory:

1. Electronic configuration of Nitrogen $(\mathrm{Z}=7)=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{px}}^{1}2{\mathrm{py}}^{1}2{\mathrm{pz}}^1.$
2. When two Nitrogen atom combine total electron present in the orbital is 14.
3. MOT combination results in the formation of ${\mathrm{N}}_2:\mathrm{KK}\text{'}{\left(\mathrm{\sigma }2\mathrm{s}\right)}^2<{(\mathrm{\sigma }*2\mathrm{s})}^2<{\left(\mathrm{\pi }2\mathrm{Px}\right)}^2={\left(\mathrm{\pi }2\mathrm{py}\right)}^2<{\left(\mathrm{\sigma }2\mathrm{pz}\right)}^2$ Figure: MOT of Nitrogen 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$\left({\mathrm{N}}_{\mathrm{a}}\right)$= 6

Number of electrons in antibonding $\left({\mathrm{N}}_{\mathrm{b}}\right)$=0

$$\begin{gathered} Bondorder=(N_b-N_a)/2 \\ =(6-0)/2 \\ =3 \end{gathered}$$

Hence, the bond order of Nitrogen molecule is 3.

Nitrogen molecule has 3 bonds. i.e. two is $\sigma$ bond and one $\mathrm{\pi }$ bond. Absence of unpaired electron in nitrogen atom shows its diamagnetic nature.