Short Answer

Valence bond theory is a model that explains how chemical bonds form between atoms by using their atomic orbitals. According to this theory, a bond is created when two atoms overlap their orbitals and share electrons. The shared electrons help both atoms achieve stability.

This theory also explains why molecules have certain shapes and bond strengths. It describes bonding in terms of orbital overlap, hybridization, and the pairing of electrons with opposite spins. Valence bond theory helps understand the nature of covalent bonds in simple molecules like H₂, CH₄, NH₃, and others.

Detailed Explanation :

Valence Bond Theory

Valence bond theory (VBT) is one of the fundamental theories used to explain how atoms form covalent bonds in molecules. It focuses on the idea that chemical bonding occurs due to the overlap of atomic orbitals. When two atoms approach each other, their orbitals—regions where electrons are likely to be found—can merge or overlap. If this overlap results in the pairing of electrons with opposite spins, a stable bond is formed. VBT was developed to provide a clearer idea of bonding behavior that early Lewis structures could not fully explain.

Valence bond theory helps describe why atoms form bonds of specific lengths and strengths, why bonds have certain directions, and why molecules adopt certain shapes. It also introduces the important concept of hybridization, which explains how atoms rearrange their orbitals to form stronger and more stable bonds.

Basic Principles of Valence Bond Theory

Valence bond theory is based on several important principles:

1. Bond Formation by Orbital Overlap

A covalent bond forms when the orbitals of two atoms overlap.
This overlap allows electrons from both atoms to share space, reducing energy and creating stability.

Greater overlap = stronger bond.
Less overlap = weaker bond.

For example:
In the H₂ molecule, the 1s orbitals of two hydrogen atoms overlap to form a strong bond.

2. Electrons Must Have Opposite Spins

A stable chemical bond requires two electrons with opposite spins.
These opposite spins reduce repulsion between electrons, allowing them to stay closer together.

Valence bond theory uses this rule to explain why only paired electrons form bonds.

3. Atomic Orbitals Retain Their Identity

In VBT, atomic orbitals do not completely lose their shape or identity when bonding occurs.
They overlap partially but still maintain their original form.

This explains why atoms have specific bonding patterns based on their available orbitals.

4. Directional Nature of Bonds

Valence bond theory explains why covalent bonds are directional.
Bonding does not happen randomly; it occurs in specific directions based on orbital orientation.

For example:

• In methane (CH₄), carbon forms four bonds arranged in a tetrahedral shape.
• This can be explained through directional overlap of hybrid orbitals.

Hybridization in Valence Bond Theory

Hybridization is one of the most significant contributions of VBT.

Hybridization means mixing of atomic orbitals to form new hybrid orbitals with equal energy.

Some common types:

• sp³ hybridization → four equal bonds (CH₄)
• sp² hybridization → three equal bonds (C₂H₄)
• sp hybridization → two linear bonds (C₂H₂)

Hybrid orbitals:

• Have better overlap
• Form stronger bonds
• Explain molecular geometry

VBT uses hybridization to describe how atoms arrange themselves to form stable molecules.

Types of Bonds Explained by VBT

1. Sigma (σ) Bonds

Sigma bonds are formed by head-on overlap of orbitals.
They are strong and allow free rotation in single-bonded molecules.

Examples:

• H–H
• C–H
• C–C in alkanes

Most single bonds in molecules are sigma bonds.

2. Pi (π) Bonds

Pi bonds form when p-orbitals overlap sideways.
They are weaker than sigma bonds and restrict rotation.

Examples:

• Second bond in O₂
• Second and third bonds in N₂
• Double and triple bonds in organic molecules

VBT clearly explains why double and triple bonds behave differently from single bonds.

Applications of Valence Bond Theory

1. Explaining Molecular Shapes

Valence bond theory helps explain shapes through hybridization:

• CH₄ → tetrahedral
• NH₃ → trigonal pyramidal
• H₂O → bent

These shapes arise from the directional overlap of orbitals.

2. Understanding Bond Strength

More overlap = stronger bond
Less overlap = weaker bond

VBT explains why:

• Triple bonds are stronger than double bonds
• Double bonds are stronger than single bonds

3. Explaining Magnetism

In O₂, VBT helps describe the bonding pattern, although molecular orbital theory gives a better explanation for paramagnetism.

Limitations of Valence Bond Theory

Although VBT is very useful, it has limitations:

• Cannot fully explain magnetic properties of molecules like O₂
• Does not accurately describe delocalisation of electrons in resonance
• Does not explain energy levels as precisely as molecular orbital theory

However, it remains a fundamental and widely used theory for understanding covalent bonding.

Conclusion

Valence bond theory explains how covalent bonds form through the overlap of atomic orbitals and the pairing of electrons with opposite spins. It describes the directional nature of bonds, the strength of bonding through orbital overlap, and molecular shapes through hybridization. Although not perfect, VBT is a powerful tool for understanding how atoms bond and why molecules form specific structures.