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Molecular Orbitals

When two hydrogen atoms come together to form the hydrogen molecule, the atomic s orbitals of each atom are combined to form two molecular orbitals. One of these new orbitals is the result of the addition of the two atomic orbitals, while the other is created by a subtraction of these orbitals. In the addition, a reinforcement of the wave function occurs in the region between the two nuclei. Physically, this means the electron density increases in the area between the two nuclei. This increase in electron density causes a corresponding increase in the attraction of each positively charged nucleus for the negatively charged overlap region. It is this increased attraction that holds the hydrogen molecule together and creates the bonding molecular orbital. Because the bonding molecular orbital is generated from atomic s orbitals, it is called a σ (sigma) bonding molecular orbital.


 The molecular orbital formed by the subtraction of the two wave functions has no electron density between the nuclei of the hydrogen atoms. This lack of electron density is caused by interference between the two out-of-phase wave functions. The lack of electron density between the nuclei results in the formation of a node. With no electron density between them, the hydrogen nuclei repel each other strongly, resulting in the formation of a high-energy state called an antibonding orbital. Because this particular antibonding orbital is created from two atomic s orbitals, it is referred to as a σ* antibonding molecular orbital