VBT: Valance Bond Theory

 Valence Bond Theory

Introduction

The valence bond (VB) theory is one of the primary theories used to describe chemical bonding. It was developed independently by Walter Heitler and Fritz London in 1927. VB theory is based on the idea that atoms share electrons to form covalent bonds. The theory assumes that the electrons in a bond are localized between the two atoms that are bonded.

VB theory is a simple and intuitive theory, and it can be used to explain a wide variety of chemical phenomena. However, VB theory has some limitations. For example, VB theory cannot easily explain the delocalization of electrons in molecules such as benzene.

Key Concepts of Valence Bond Theory

• Atomic orbitals: Atomic orbitals are the regions of space around an atom where there is a high probability of finding an electron. There are four types of atomic orbitals: s, p, d, and f.
• Hybridization: Hybridization is the mixing of atomic orbitals to form new hybrid orbitals. Hybridization occurs when atoms form covalent bonds.
• Sigma bonds (σ bonds): Sigma bonds are formed when two atomic orbitals overlap head-to-head. Sigma bonds are strong bonds.
• Pi bonds (π bonds): Pi bonds are formed when two atomic orbitals overlap side-by-side. Pi bonds are weaker than sigma bonds.
• Multiple bonds: Multiple bonds are formed when two atoms share more than one pair of electrons. Double bonds are formed when two atoms share two pairs of electrons, and triple bonds are formed when two atoms share three pairs of electrons.

Valence Bond Theory and Hybridization

Hybridization is a key concept in VB theory. Hybridization occurs when atomic orbitals mix to form new hybrid orbitals. The hybrid orbitals are then used to form covalent bonds.

There are several different types of hybridization:

• sp hybridization: sp hybridization occurs when one s orbital and one p orbital mix to form two sp hybrid orbitals. sp hybridization is used to form linear molecules, such as carbon dioxide (CO2).
• sp2 hybridization: sp2 hybridization occurs when one s orbital and two p orbitals mix to form three sp2 hybrid orbitals. sp2 hybridization is used to form trigonal planar molecules, such as methane (CH4).
• sp3 hybridization: sp3 hybridization occurs when one s orbital and three p orbitals mix to form four sp3 hybrid orbitals. sp3 hybridization is used to form tetrahedral molecules, such as water (H2O).

Valence Bond Theory and Multiple Bonds

VB theory can also be used to explain multiple bonds. Double bonds are formed when two atoms share two pairs of electrons. One pair of electrons forms a sigma bond, and the other pair of electrons forms a pi bond. Triple bonds are formed when two atoms share three pairs of electrons. One pair of electrons forms a sigma bond, and the other two pairs of electrons form two pi bonds.

Valence Bond Theory and Resonance

Resonance is a concept in VB theory that is used to explain the delocalization of electrons in molecules. Resonance occurs when a molecule can be represented by two or more Lewis structures that differ only in the placement of electrons. The actual structure of the molecule is a hybrid of the resonance structures.

For example, benzene can be represented by two resonance structures. In one resonance structure, the six carbon atoms are connected by alternating single and double bonds. In the other resonance structure, the six carbon atoms are connected by alternating double and single bonds. The actual structure of benzene is a hybrid of these two resonance structures.

Limitations of Valence Bond Theory

VB theory is a simple and intuitive theory, but it has some limitations. For example, VB theory cannot easily explain the delocalization of electrons in molecules such as benzene. In addition, VB theory can be difficult to apply to molecules that have complex geometries.

Molecular Orbital Theory

Molecular orbital (MO) theory is another theory that is used to describe chemical bonding. MO theory is more complex than VB theory, but it can provide a more accurate description of chemical bonding.

MO theory is based on the idea that electrons are delocalized over the entire molecule. MO theory assumes that the electrons in a molecule occupy molecular orbitals, which are formed by the combination of atomic orbitals.

MO theory can be used to explain a wide variety of chemical phenomena, including the delocalization of electrons in molecules such as benzene. However, MO theory is more difficult to apply than VB theory.

Conclusion

Valence bond theory is a simple and intuitive theory that can be used to explain a wide variety of chemical phenomena. However, VB theory has some limitations, such as its inability to easily explain the delocalization of electrons in molecules. Molecular orbital theory is a more complex theory that can provide a more accurate description of chemical bonding.

Overview

1. Introduction to Valence Bond Theory
2. Key Concepts of Valence Bond Theory
3. Valence Bond Theory and Hybridization
4. Valence Bond Theory and Multiple Bonds
5. Valence Bond Theory and Resonance
6. Limitations of Valence Bond Theory
7. Molecular Orbital Theory
8. Conclusion

Additional Resources

• Atkins, P. W. (2010). Physical Chemistry (9th ed.). Oxford University Press.
• McQuarrie, D. A., & Simon, J. D. (1997). Physical Chemistry: A Molecular Approach (1st ed.). University Science Books.
• Zumdahl, S. S. (2009). Chemistry: An Atoms First Approach (4th ed.). Cengage Learning.

This seminar outline can be used as a starting point for a 5-7 page seminar on valence bond theory. The additional resources can be used to provide more information on the topic.