VALENCE BOND THEORY

Valence Bond Theory (VB – T) Introduced by Heilter and London (1927), after which this theory also known as London Forces. Based on Atomic Orbital Concept and E.C. of atoms. Was further developed by Nobel laureate Prof. Linus Pauling. The model tells us that Covalent Bond is formed when two orbital of the combining atoms overlap to produce a new combined orbital containing 2 e – of opposite spins. The resulting overlapping results in the decrease in the Energy of the atoms forming the bond. The shared e pair is most likely to be found in the space between the 2 nuclei of the combining atoms.

Example of H 2 molecule using VB – T s – s Overlapping

More Explanation on Orbital Overlapping Concept (taking H 2 as example) As two nuclei of the 2 H2 atoms come closer for the formation of a bond, new attractive and repulsive forces begin to operate. The nucleus of one atom is attracted (Attractive Forces) towards its own electron and the electron of the other atom and vice versa. Repulsive Forces arise between the e – of the 2 atoms and the nuclei of the 2 atoms. However, Attractive Forces are more compared to Repulsive Forces, hence, the atoms come together forming a bond. The H – H bond energy of kJ/mol makes it very stable compared to single H atom.

According to Orbital Overlapping Concept, when atoms combine and overlap to form covalent bond, they possess e – having opposite spins. Formation of HF Bond by combination of 1 F and 1 H atom: Z of F = 9, E.C. of F = 1s 2 2s 2 2p x 2 2p y 2 2p z 1 Z of H = 1, E.C. of H = 1s 1

Types of Orbital Overlap Depending on the type of overlapping, the covalent bonds are of the following types: 1. Sigma Bond ( σ) 2. Pi Bond ( π) Sigma Bond (σ): (Formed by Axial Overlapping) Is formed by end to end overlapping (head – on overlap) of bonding orbital along the interneuclear axis. These are of 3 types: 1. s – s overlapping: in this case there is overlapping of 2 half – filled s – orbitals along the internuclear axis

2. s – p overlapping: in this case there is overlapping between half filled s – orbital of one atom and half – filled p – orbital of the other atom. 3. p – p overlapping: in this case there is overlapping of half – filled p – orbitals of the two approaching atoms.

Pi Bond (π): Sideways Overlapping Is formed by the overlapping of atomic orbitals when they overlap in a way that their axes remain parallel to each other and perpendicular to the internuclear axis.

Hybridization It’s the process of intermixing of the orbitals (Hybrid) of the combining atoms of slightly different energies. They form different shapes of orbitals after combining. Orbitals of Valence Shells of almost similar energies can participate. Number of hybrid orbitals produced depends on the number of atomic orbitals participating. Geometry of the molecule formed can be determined by its type of Hybridization. The hybrid orbitals are more effective in forming stable bonds than pure atomic orbitals. Exceptionally, in some cases filled orbitals of valence shell also sometimes take part in hybridization.

Types of Hybridization sp Hybridization: When 1 s and 1p orbital hybridize to form 2 equivalent orbitals, it is known as sp Hybridization. Each of the 2 hybrid orbitals will possess 50% s character and 50% p character. Ex: Acetylene (C 2 H 2 ), BeCl 2, BeF 2, BeH 2 etc.

sp 2 Hybridization: When 1 s and 2 p orbitals hybridize to form 3 equivalent orbitals, it is known as sp 2 Hybridization. All the 3 hybrid orbitals remain in the same plane making an angle of Eg: Compounds like BF 3, BH 3, BCl 3 etc. and those compounds containing Carbon Carbon double bonds.

sp 3 Hybridization: When 1 s and 3 p orbitals hybridize in the valence shell to form 4 equivalent hybrid orbitals, it is known as sp 3 Hybridization. There is 25% s character and 75% p character in each sp 3 hybrid orbital. The four hybrid orbitals remain directed towards the four corners of a regular tetrahedron. It generally makes an angle of Eg: Compounds like NH 3 and H 2 O exhibit such hybridization.

Hybridization involving s, p, and d orbitals There are 2 such types: sp 3 d Hybridization: When 1 s, 3 p and 1 d orbitals of the valence shells are involved in hybridization, it gives rise to 5 equivalent orbitals. Out of the 5, 3 are directed towards the corners of an equilateral triangle, whereas, 2 of them are perpendicular to the plane of the triangle. The former 3 create an angle of Eg: PCl 5 molecule behave much the same way as sp 3 d Hybridization.

sp 3 d 2 Hybridization: When 1 s, 3 p and 2 d orbitals of the valence shells hybridize, it gives 6 new equivalent hybrid orbitals, which are projected towards the 6 corners of a regular octahedron. For example, molecules like SF 6 behave in much the same way.

sp 3 d 3 Hybridization: When 1 s, 3 p and 3 d orbitals of the valence shell hybridize, it forms 7 new equivalent hybrid orbitals, which are projected towards the seven corners of a regular pentagonal bipyramid. Eg: Molecules like IF 7 exhibit such geometry.