1.6 Molecular Orbital Theory

According to molecular orbital theory, covalent bonds result from the combination of two atomic orbitals to form molecular orbitals. For example, in hydrogen molecule the 1s atomic orbital of one hydrogen atom overlaps with the 1s atomic orbital of a second hydrogen atom to give a molecular orbital. The covalent bond that is formed when the two s atomic orbitals overlap is called a cylindrically symmetrical σ -bond .

• During bond formation, energy is released as the two orbitals start to overlap. More energy released as increases the overlap of orbitals. When the atoms approach each other, their positively charged nuclei repel each other. This repulsion causes a large increase in

Figure 1

energy. We see that maximum stability is achieved when the nuclei are a certain distance apart. This distance is the bond length of the covalent bond which is 0.74 Å for hydrogen molecule (Figure 1). When the bond forms, 104 kcal/mol of energy is released and the same amount of energy has to be given for breaking the bond. The energy required to break a bond is called the bond dissociation energy .

• Atomic orbitals can combine in two different ways. For example, the 1s orbitals of two hydrogen atoms approach each other and combine constructively, when they are in phase and have the same sign, to give a σ bonding molecular orbital (Figure 2).
  
  



Figure 2