Answer to Question #141932 in Inorganic Chemistry for Olebogeng

Question

For early period homonuclear diatomic molecules, the 2π orbitals are lower in energy than the 2σ orbital. However, for late period homonuclear diatomic molecules, the reverse is true. Explain why this is so

Answer 1

Assuming our focus is in period 2 elements, the early period may represent elements from Li₂ to N₂, while late period consists of elements from O₂ to F₂. The difference that exists between between these two groups is due to the different arrangement in their molecular orbitals.

From L₂ to N₂, for instance, 2$\pi$ molecular orbitals are lower in energy than 2$\sigma$ orbital because there is less separation between the 2s and 2p atomic orbitals (the two are similar in energy). However, further across the period, this separation between 2s and 2p becomes greater and thus decreasing the stability of the 2$\pi$ orbitals increasing their energy relative to 2$\sigma$ orbitals.

According to quantum Mechanics, the more similar are two orbitals, the better they mix resulting in more stability and less energy.