Choose the correct order of energy of 2σ_g and 1π_u molecular orbitals for B₂, C₂ and O₂:

Concept:

Molecular Orbital Theory:

• The molecular orbital theory predicts that each atom tends to combine with each other and results in the molecular orbital. As a result, electrons are found in various atomic orbitals and they are usually associated with different nuclei.

Explanation:

• According to molecular orbital theory, the energy of molecular orbital increases in the order 

1σg < 1σ^*u < 1π_u < 2σg < 1π^*_g < 2σ^*_u (neglecting 1s orbital) for the elements having the number of electrons less than or equal to 14.

• For the element B₂ and C₂, the total number of electrons are 10 and 12 as each B and C atom contributes 5 and 6 electrons respectively. So, the correct order of energy of 2σg and 1πu molecular orbitals for B2 and C2 are 

2σg > 1πu. 

• For an O₂ molecule, the total number of electrons is 16, as each O atom contributes 8 electrons. The energy of molecular orbital for O2 molecule increases in the order 

1σg < 1σ*u < 2σg < 1πu < 1π*g < 2σ*u .

• Hence, for the O2 molecule, the order of energy of 2σg and 1πu  is 

2σg < 1π_u.

Conclusion:

• The correct order of energy of 2σg and 1πu molecular orbitals for B2, C2, and O2: 

2σg > 1πu for B2 and C2 only.