Sigma bonding molecular

If the two atomic orbitals are s-orbitals then the resulting molecular orbitals are called o (sigma) bonding molecular orbitals and a (sigma star) antibonding molecular orbitals (Figure 1.5). 

The superscript in indicates that there is one electron in the sigma bonding molecular orbital. 

In forming the N2 from N2, an electron is removed from the sigma bonding molecular orbital. Consequently, the bond order decreases to 2.5 from 3.0. In forming the OJ ion from O2, an electron is removed from the pi antibonding molecular orbital. Consequently, the bond order increases to 2.5 from 2.0. 

It is the wavelike properties of the electrons that cause two atomic orbitals to form two molecular orbitals. Two atomic orbitals can combine in an additive (constractive) manner, just as two light waves or two sound waves can reinforce each other (Figure 1.3a). The constructive combination of two s atomic orbitals is called a a (sigma) bonding molecular orbital. 

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