Answer:
Orbital mixing or sigma pi crossover
Explanation:
Looking at the diagram below, the ordering of molecular orbitals differ between carbon and oxygen molecules. The reason for this is that, with increase in atomic number, the energies of sigma 2p bonding molecular orbital and pi 2p molecular orbital come close together but are energetically far apart in the lighter elements. Following this ordering of orbitals, the two degenerate pi- 2p antibonding orbitals are singly filled accounting for the paramagnetism of oxygen while the two degenerate pi-2p bonding orbitals in carbon molecule is doubly occupied hence the molecule is diamagnetic.
