Molecular Orbital Theory for the Formation of Covalent Bond.
According to this theory a covalent bond is formed when two half-filled orbitals of the two atoms come nearer and then overlap each other to form a new bigger orbital, known as molecular orbital (MO). The molecular orbital surrounds the atomic nuclei of both the atoms and each of the electrons, which are now paired in the molecular orbital, is electrostatically attracted by both the nuclei. It is important to note that energy of the MO is less than the sum of energies of the two atomic orbitals and consequently the resulting molecule is more stable than the two separate atoms. Like atomic orbitals, MOS also obey Pauli's exclusion principle and thus one MO can accommodate only two electrons with antiparallel spins. When two atomic orbitals overlap each other, two MOS are produced, namely bonding (having lesser energy than the energies of the separate atomic orbitals) and antibonding (having higher energy than the energies of the two separate atomic orbitals). These two new MOS spread over both the atoms and either may contain two electrons.
If overlap of the two atomic orbitals takes place along their axes, the resulting bonding MOs are known as sigma (σ) orbitals and the bond formed as a o bond. On the other hand, if overlapping of the two atomic orbitals takes place sideways, the resulting MOS are known as pi (π) orbitals and the bond formed by them as л bоnd. л-Воnd is formed by the overlapping of py - py and pz - pz orbitals.
Molecular orbitals formed by the overlapping of 1s, 2s, 2px, 2py and 2pz orbitals are arranged below in the order of increasing energy
σ 1s, σ* 1s, σ 2s, σ* 2s, σ 2рx, π 2рy ≡ π 2рz, π* 2ру ≡ π* 2рz, σ*2рx
The energy levels of different molecular orbitals are shown diagrammatically in Fig. However, experimental evidences suggest that σ 2px MO is higher in energy than that of π 2рy and π 2рz MOS and thus the new molecular orbital energy level diagram is adopted for writing MO configurations of homonuclear diatomic molecules of elements upto atomic number 7 i.e. for H2, He2 (hypothetical), Li2, Be2 (hypothetical), B2, C2 and N2. This is because of the fact that in the atoms of these molecules, the 2s and 2px atomic orbitals lie fairly close together, i.e. 2s - 2py energy difference is quite small, and therefore 2s - 2pz mixing is quite large.
