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-1 bond length 120.8 pm
To understand why the bond in N2 is stronger and shorter compared to O2, we need to consider molecular orbital theory and examine the appropriate molecular orbital diagrams.
Molecular orbital theory states that when two atoms combine to form a molecule, the atomic orbitals of the individual atoms combine to create new molecular orbitals. These molecular orbitals are occupied by electrons. The bonding molecular orbitals, which have lower energy than the original atomic orbitals, contribute to bond formation, while the antibonding molecular orbitals, which have higher energy than the original atomic orbitals, contribute to bond weakening.
In the case of N2, let's consider the molecular orbital diagram. Nitrogen has 7 electrons, so when two nitrogen atoms combine, they contribute a total of 14 electrons.
The molecular orbital diagram for N2 is as follows:
σ2p* (anti-bonding) (↑↓)
σ2p (bonding) (↑↓)
π2p (anti-bonding) (↑↓)
σ2s* (anti-bonding) (↑↓)
σ2s (bonding) (↑↓)
In the molecular orbital diagram, the bonding molecular orbitals are denoted with the symbol σ, while the antibonding molecular orbitals are denoted with σ*.
In N2, there are two bonding electrons in the σ2p orbital, which contribute to bond formation, resulting in a strong bond. Additionally, the bond length is shorter because there is a greater electron density between the nitrogen atoms, leading to a stronger attractive force holding the atoms together.
On the other hand, let's consider the molecular orbital diagram for O2. Oxygen has 8 electrons, so when two oxygen atoms combine, they contribute a total of 16 electrons.
The molecular orbital diagram for O2 is as follows:
σ2p* (anti-bonding) (↑↓)
σ2p (bonding) (↑↓)
π2p* (anti-bonding) (↑↓)
σ2s* (anti-bonding) (↑↓)
σ2s (bonding) (↑↓)
In O2, there are two bonding electrons in the σ2p orbital, contributing to bond formation. However, there are also two antibonding electrons in the π2p* orbital, which contribute to bond weakening. This results in a weaker bond compared to N2. Additionally, the bond length is longer because there is a lower electron density between the oxygen atoms, leading to a weaker attractive force.
In summary, the bond in N2 is stronger and shorter because there are more bonding electrons and a higher electron density compared to O2. Molecular orbital theory allows us to understand the electronic structure of molecules and explains the differences in bond strength and length between N2 and O2.