The Central Themes of VB Theory

The basic principle of VB theory is that a covalent bond forms when orbitals of two atoms overlap and the overlap region, which is between the nuclei, is occupied by a pair of electrons. ( Orbital overlap is another way of saying that the two wave functions are in phase, so the amplitude increases between the nuclei.) The central themes of VB theory derive from this principle  

Opposing spins of the electron pair. As the exclusion principle (Section 

Hybridization of atomic orbitals. To account for the bonding in simple diatomic molecules like HF, we picture the direct overlap of 5 and p orbitals of isolated atoms. But how can we account for the shapes of so many molecules and polyatomic ions through the overlap of spherical s orbitals, dumbbell-shaped p orbitals, and cloverleaf-shaped d orbitals  

Consider a methane molecule, CH4. It has four H atoms bonded to a central C atom. An isolated ground-state C atom ([He] 2s 2p ) has four valence electrons two in the 2s orbital and one each in two of the three 2p orbitals. We might easily see how the two half-filled p orbitals of C could overlap with the ]s orbitals of two H atoms to form two C—H bonds with a 90° H—C—H bond angle. But methane is not CH2 and doesn t have a bond angle of 90°. It s not as easy to see how the orbitals overlap to form the four C—H bonds with the 109.5° bond angle that occurs in methane. 

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Valence Bond (VB) Theory and Orbital Hybridization 329 The Central Themes of VB Theory 329... 

The central theme of this chapter is to show how the many attractive physical concepts of qualitative VB theory can be incorporated into a more general framework (the spin-coupled valence bond theory, Section III) which leads to a computational procedure that provides quantitative descriptions of the ground-state and many excited-state potential energy surfaces of molecular systems. 

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