Molecular orbital theory anti-bonding orbitals

Group theory was used to define the orbitals of the dihydrogen molecule. This led to the molecular orbital diagram for H2 and the relative energies of atomic, bonding and anti-bonding orbitals. 

As predicted by molecular orbital theory dioxygen has two unpaired electrons and some of its chemistry shows diradical characteristics in particular, it reacts readily with other radicals. Singlet oxygen is an excited state in which the two electrons in the p anti-bonding orbitals have paired spins. It produced in some chemical reactions and has different chemical reactivity. 

Modern bonding theory treats the electron pairing in terms of the interaction of electron (atomic) orbitals and describes the covalent bond in terms of both bonding and anti-bonding molecular orbitals. 

This reaction proceeds, according to perturbation theory, by electrons moving from occupied bonding, or non-bonding, orbitals into empty, anti-bonding orbitals. Alternatively the anti-bonding orbitals are mixed with the bonding orbitals to form new molecular orbitals. 

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