1. Valence Electrons: Valence Bond Theory (VBT) is a model that helps us understand how chemical bonds are formed. It focuses on the interactions between the valence electrons of atoms.
2. Atomic Orbitals: In VBT, atomic orbitals are the regions around an atomic nucleus where electrons are likely to be found. The most common atomic orbitals are s, p, d, and f orbitals.
3. Hybridization: Hybridization is a concept that explains the mixing of atomic orbitals to form new hybrid orbitals with different shapes and energies. It occurs when atoms combine to form chemical bonds.
4. SP Hybridization: When one s and one p orbital mix, they form two SP hybrid orbitals. This is commonly seen in molecules like CH4 (methane).
5. SP2 Hybridization: When one s and two p orbitals mix, they form three SP2 hybrid orbitals. This is observed in molecules like C2H4 (ethylene).
6. SP3 Hybridization: When one s and three p orbitals mix, they form four SP3 hybrid orbitals. This is found in molecules like CH3OH (methanol).
7. Sigma (σ) Bonds: The overlap of two hybrid orbitals or an orbital and a non-hybridized orbital forms a sigma bond. Sigma bonds are strong and allow for free rotation.
8. Pi (π) Bonds: Pi bonds are formed by the side-to-side overlap of p orbitals. They are weaker than sigma bonds and restrict rotation.
9. Multiple Bonds: Double bonds consist of one sigma bond and one pi bond, while triple bonds have one sigma bond and two pi bonds.
10. Geometry of Molecules: Hybridization affects the geometry of molecules. For example, SP3 hybridization leads to a tetrahedral geometry, SP2 hybridization results in a trigonal planar geometry, and SP hybridization leads to a linear geometry.
11. Application: Understanding hybridization helps explain the shapes and properties of molecules and is crucial in organic chemistry, where the arrangement of atoms in a molecule can significantly affect its reactivity and behavior.
12. Limitations: While Valence Bond Theory is useful for explaining the formation of chemical bonds, it has limitations and does not provide a complete description of molecular bonding. Molecular Orbital Theory is another important theory that complements VBT and provides a more accurate description of bonding in molecules.

Postulates of Valence Bond Theory

The important postulates of the valence bond theory are listed below.

Coordination Number	Type of Hybridisation	Distribution of Hybrid Orbitals in Space
4	sp3	Tetrahedral
4	dsp2	Square planar
5	sp3d	Trigonal bipyramidal
6	sp3d2	Octahedral
6	d2sp3	Octahedral

1. Covalent bonds are formed when two valence orbitals (half-filled) belonging to two different atoms overlap on each other. The electron density in the area between the two bonding atoms increases as a result of this overlapping, thereby increasing the stability of the resulting molecule.
2. The presence of many unpaired electrons in the valence shell of an atom enables it to form multiple bonds with other atoms. The paired electrons present in the valence shell do not take participate in the formation of chemical bonds as per the valence bond theory.
3. Covalent chemical bonds are directional and are also parallel to the region corresponding to the atomic orbitals that are overlapping.
4. Sigma bonds and pi bonds differ in the pattern that the atomic orbitals overlap in, i.e. pi bonds are formed from sidewise overlapping whereas the overlapping along the axis containing the nuclei of the two atoms leads to the formation of sigma bonds.

It can be noted that sigma bonds involve the head-to-head overlapping of atomic orbitals whereas pi bonds involve parallel overlapping.

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