Valence Bond Theory (VB theory) is a model in chemistry that explains chemical bonding in terms of overlapping atomic orbitals. It was one of the first theories developed to describe how atoms combine to form molecules by sharing electrons. Valence Bond Theory provides a qualitative understanding of covalent bonding and the shapes of molecules based on the interactions between atomic orbitals.

Key concepts of Valence Bond Theory include:

1. Atomic Orbitals: Atomic orbitals are regions of space around an atom’s nucleus where electrons are likely to be found. These orbitals have different shapes and energies.
2. Covalent Bond Formation: According to VB theory, a covalent bond is formed when two atoms come close together and their atomic orbitals overlap. The overlapping orbitals share electrons, resulting in the formation of a bond.
3. Hybridization: VB theory introduced the concept of hybridization, where atomic orbitals of the same atom combine to form new hybrid orbitals that are suitable for overlapping with orbitals of other atoms. Hybrid orbitals are used to explain the geometry of molecules.
4. Sigma (σ) and Pi (π) Bonds: Valence Bond Theory distinguishes between sigma (σ) bonds and pi (π) bonds. Sigma bonds are formed by the direct head-on overlap of atomic orbitals, while pi bonds result from the lateral overlap of p orbitals.
5. Overlapping Orbitals: Overlapping atomic orbitals can have different orientations, leading to various types of bonds and molecular geometries.
6. Resonance: VB theory can explain resonance in molecules with multiple equivalent bond arrangements by considering the mixing of different contributing resonance structures.
7. Limitations: While Valence Bond Theory provides a useful qualitative understanding of bonding and shapes of molecules, it has limitations in explaining more complex molecular structures and properties. Molecular Orbital Theory (MO theory) is another theory that addresses some of these limitations.

Valence Bond Theory played a significant role in the development of our understanding of chemical bonding and served as a foundation for the later development of more advanced theories, including Molecular Orbital Theory. It’s a valuable tool for explaining the basic principles of covalent bonding and molecular structure.