By reference to Molecular orbital Theory and giving the appropriate MO diagrams explain why the bond in N2 is very strong short (bond dissociation entalpy 945 kjmol-1,bond length 109.8 pm) while that in O2 is weaker and longer (bond dissociation entalpy 497 kJ mol bond length 120 .8 pm

Question

By reference to Molecular orbital Theory and giving the appropriate MO diagrams explain why the bond in N2 is very strong short (bond dissociation entalpy 945 kjmol-1,bond length 109.8 pm) while that in O2 is weaker and longer (bond dissociation entalpy 497 kJ mol bond length 120 .8 pm

Answer 1

To explain the difference in bond strength and length between N2 and O2 using Molecular Orbital (MO) Theory, we need to examine the MO diagrams for each molecule. MO Theory describes the formation of molecular orbitals from the overlap of atomic orbitals. In these diagrams, the molecular orbitals are represented by combining the atomic orbitals of the atoms involved.

Let's start with the MO diagram for nitrogen (N2):

N ≡ N

In the nitrogen molecule, each nitrogen atom contributes three atomic orbitals: 2pσ, 2pπx, and 2pπy. These atomic orbitals combine to form a set of six molecular orbitals: σ2p, σ*2p, π2p, π*2p, π2p, and π*2p.

The strongest and shortest bond in N2 is the σ2p bonding orbital. This bond is formed by the overlap of the two 2pσ orbitals from each nitrogen atom. This overlap is very effective since the orbitals are oriented head-on, resulting in a strong bond. The bond dissociation enthalpy of 945 kJ/mol and bond length of 109.8 pm reflect this strong interaction.

Now let's move on to the MO diagram for oxygen (O2):

O = O

Each oxygen atom in O2 contributes two atomic orbitals: 2pσ and 2pπ. These atomic orbitals combine to form four molecular orbitals: σ2p, σ*2p, π2p, and π*2p.

In contrast to N2, the bonding in O2 involves overlap of only the 2pπ orbitals. The σ2p bonding orbital in O2 is not particularly strong due to the side-to-side orientation of the atomic orbitals, resulting in weaker overlap. As a result, the bond in O2 is weaker with a bond dissociation enthalpy of 497 kJ/mol and a longer bond length of 120.8 pm.

So, in summary, the stronger and shorter bond in N2 is a result of the effective head-on overlap of the 2pσ orbitals, while the weaker and longer bond in O2 stems from the weaker side-to-side overlap of 2pπ orbitals in the MO diagram.