Theories of Covalent Bonding and Shapes of Molecules: Short questions and answers

Question: Energies of orbitals can be explained by molecular orbital theory. It has been observed that in case of nitrogen molecules 2px is higher in energy than π_2Py and π_2Pz.

Answer.

In case of N, the energy gap between 2s and 2p atomic orbitals (AOs) is small. So, during formation of N₂, these AOs intermix with each other. As a result molecular orbitals (MO), σ_2s and σ*_2s do not retain pure s character and MO σ_2Px and σ*_2Px do not retain pure p character.  These entire MOs attain sp character. Due to this intermixing of atomic orbitals, σ2s and σ2s* becomes lower in energy and hence more stable while σ_2Px and σ*_2Px become higher in energy and hence less stable.

Since, π_2Py and π_2Pz orbitals do not involve in intermixing, therefore energy of these orbitals remain the same. So, σ_2Px orbital becomes higher in energy than π_2Py and π_2Pz orbitals. 

Question: Draw energy diagram for N₂ molecules

Question: Molecular orbital theory explain the paramagnetic character of O₂, O₂²⁺ and O₂^2- species. Evaluate it.

Answer.

Molecular orbital diagram of O₂  has been shown as  follows.

Formation of O₂

O₂ has two unpaired electrons in π_2py and π_2pz orbitals. Due to presence of these unpaired electron O₂ is attracted by magnetic field, so it is paramagnetic substance.

Formation of O₂²⁺

Molecular orbital diagram of O₂²⁺ has been shown as  follows.

O₂²⁺ has no unpaired electrons . Due to absence of these unpaired electron, O₂²⁺ is not attracted by magnetic field, so it is diamagnetic substance.

Formation of O₂^2-

Molecular orbital diagram of O₂^2- has been shown as  follows.

O^2- has  no unpaired electrons . Due to absence of these unpaired electron O^2- is not attracted by magnetic field, so it is diamagnetic substance.

Question: Distinguish between a sigma bond  and a pi bond.

Answer.

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