What is the correct order of relative energy for molecular orbitals?

The correct order of relative energy for molecular orbitals is indeed reflected in the provided answer. In diatomic molecules, molecular orbital (MO) theory provides a framework to predict the energies and the order of these orbitals based on their interactions and symmetry.

Starting with the orbitals mentioned, the molecular orbitals can be categorized based on their angular momentum and bonding characteristics:

• The sigma (σ) orbitals are lower in energy compared to pi (π) orbitals because σ orbitals are formed from the head-on overlap of atomic orbitals, which provides stronger bonding interactions.

• Moving up the energy scale: the 1σ orbital, which is the lowest energy molecular orbital formed from the overlap of two s-orbitals, is followed by the 2σ, 3σ, and 4σ orbitals. The numerical designation corresponds to their increasing energy levels, with 4σ being the highest energy amongst the σ orbitals mentioned.

• The π orbitals, arising from the side-on overlap of p-orbitals, are generally higher in energy than the σ orbitals of the same principal quantum number. Hence, for example, 2π is lower in energy than the 3σ but higher than the 1σ