Valence Bond Theory and hybridization are the very very important components for understanding the nature of chemical bonding in molecules and compounds. This theory is used to explain how atoms share electrons in covalent bonds through the overlapping of their atomic orbitals. The principles of Valence Bond Theory allow a person to understand what hybridization is all about-which is simply a mixture of atomic orbitals into new hybrid orbitals. This article intends to explain Valence Bond Theory and hybridization in detail, unravelling their significance in the prediction of molecular structures, explanations of strengths of bonds, and the elucidation of reactivity of chemical species.
Table of Contents
Introduction to Valence Bond Theory
Valence Bond Theory is a foundational concept in chemistry explaining how atoms make up chemical bonds by the sharing of electrons.
Definition of Valence Bond Theory :
According to the Valence Bond Theory, a chemical bond is the formation as a result of two atoms approaching each other to overlap their orbitals, thus resulting in the sharing of electrons.
VBT Origin :
Valence Bond Theory was 1st proposed by Linus Pauling in 1931. He explained the chemical bonding phenomenon better than previous theories.
Concept of Chemical Bonding :
Chemical bonding is the phenomenon or an interaction of two or more atoms that formed or produces compounds. Covalent bonds are one such chemical bond where electrons are shared among atoms.
Covalent Bonds :
Covalent bonds result when atoms share electrons (on the sharing electrons both atoms have dominance) for an atom to achieve a stable electron configuration. This sharing of electrons creates a bond which can be single, double or triple bond that holds the atoms together in a molecule.
Electronegativity and Bond Polarity :
The relative ability of an atom to attract shared electrons in a chemical bond is known as electronegativity. The difference in electronegativity between Two or more atoms determines the polarity of a bond, that is, how much an atom can attract electrons from another more electronegative atom.
Valence Bond Theory Principles :
The principles of Valence Bond Theory concern overlapping atomic orbitals to form various types of bonds.
Overlap of Atomic Orbitals :
In Valence Bond Theory, overlap of atomic orbitals from different atoms leads to sharing of electrons and thereby giving rise to a covalent bond.
Sigma and Pi Bonds :
Overlapping of two atomic orbitals by head-to-head (horizontallly) lead to the formation of sigma bonds. Sigma bonds are quite strong and directional. Pi bonds arise from side-by-side overlap of p-orbitals and contribute towards the stability of molecules.
What is Hybridization in Chemical Bonding :
Hybridization is very most important aspect to understanding the molecular shape and the properties of the molecules.
Enhancement of Bonding Predictions :
Hybridization permits one to predict what types of bonds and shapes one might expect for molecules, a way of knowing chemical properties through the choice that atoms will make to adopt in forming a compound.
Molecular Structure Stability :
Hybridization helps stabilize molecular shapes as it allows atoms to adopt the most favorable electron orbitals that contribute to the overall stability of molecules.
Types of Hybridization :
Atoms normally hybridize in the formation of molecules. Hybridization is the phenomenon results in the production of new orbitals.
sp Hybridization
During sp hybridization, one s and one p orbital combine to form sp (sp) hybrid orbitals. These both orbitals are situated linear and are opposite oriented to each other at an angle of 180 degrees. Hybridization of this type most commonly occurs in the case of molecules like BeCl2 and acetylene or C2H2, a linear molecule.
sp2 Hybridization
sp2 Hybridization is formed by the overlapping of a one s orbital with two p orbitals to result in three sp2 hybrid orbitals arranged trigonally at 120 degrees angles to each other. This is observed as hybridization in molecules like BF3 and C2H4.
sp3 Hybridization
sp3 hybridization is achieved due to the overlap of one s orbital and three p orbitals that lead to the creation of four sp3 hybrid orbitals. These are arranged tetrahedrally at 109.5 degrees to each other. The most common examples of molecules that undergo sp3 hybridization include methane, CH4 and ethane, C2H6.
Hybridization in Molecules and Compounds :
Hybridization in Methane (CH4)
In the case of methane, there are sp3 hybridization between each carbon and hydrogen atom and four equivalent sp3 hybrid orbitals are formed by sigma bond. Each one overlaps with a 1s orbital of a hydrogen atom, thereby giving the carbon atom a tetrahedral shape around it.
Hybridization in Ethene (C2H4)
In ethylene (C2H4), each carbon atom undergoes sp2 hybridization due to one pi bond formation, hence producing three sp2 hybrid orbitals. Two of these orbitals overlap with the 1s orbital of hydrogen atoms, and the third one forms a pi bond with the other carbon atom. The product of this process is the trigonal planar structure having a double bond between carbon atoms.
Applications of VBT and Hybridization :
Prediction of Molecular Geometries
The valence bond theory along with hybridization can predict the shapes of molecules. It can be understood that knowing the kind of hybridization may determine whether the molecule would be linear or trigonal planar or tetrahedral in geometry.
Explain Chemical Reactivity
Hybridization has also appeared as an important factor to explain chemical reactivity of molecules. The nature of the bonds formed and the geometry that the atoms adopt influence how molecules react with other substances, undergo further reactions, and bond with other elements.
Valence Bond Theory vs. Molecular Orbital Theory:
While valence bond theory explains chemical bonding within the molecules by using the concept of hybridization, molecular orbital theory explains it by the formation of molecular orbitals resulting from the linear combination of atomic orbitals. Localized electron pairs and bonds have their importance in Valence bond theory, while molecular orbital theory talks about delocalized electrons and overall distribution of the electron in the molecule.
The shortcomings of valence bond theory
Here are the step-by-step explanations concerning the limitations of Valence Bond Theory, or VBT.
1. Applies to Simple Molecules Only: Valence Bond Theory can be applied quite effectively only to simple molecules like HCl, CCl4 etc. where the concept of overlapping atomic orbitals is enough to explain the formation of bonds. However, for such complex molecules containing multiple resonance structures and delocalized Pi electrons, this theory is not so efficient.
2. Violates the principle of Molecular Orbital Theory (MOT) : Valence Bond Theory is founded on localized bonds, which result from overlap among atomic orbitals. It has no consideration for molecular orbitals, which occur due to the combination of atomic orbitals throughout a molecule. Molecular Orbital Theory explains delocalization of the electron more vividly and generally yields a better account of stability in a molecule.
3. Failure in Predicting Molecular Geometries: Valence Bond Theory sometimes fails to predict the geometries of molecules quite accurately, especially when there exist resonance structures for that molecule or the molecule contains transition metal complex molecules. Molecular geometries are better explained and predicted by the concepts employed in Molecular Orbital Theory.
4. Does not Explain Magnetic Properties :
Valence Bond Theory cannot account for the magnetic properties of molecules, such as paramagnetism and diamagnetism. In this aspect, its performance is supplemented well by Molecular Orbital Theory. In that case, as noted above, the entire molecular electron configuration is considered.
5. Failure to Describe π-Conjugated Systems:
The Valence Bond Theory fails as a description of π-conjugated systems in which delocalized π-electrons greatly contribute to the stability and reactivity of molecules. The phenomenon of aromaticity, antiaromaticity, and anything involving the conjugated system is simply left unexplained.
6. Transition metal complexes: Valence Bond Theory fails to explain the bonding in transition metal complexes. The complex bonding interactions involving d- orbitals are more suitably explained by Ligand Field Theory and Crystal Field Theory that is based on principles of Molecular Orbital Theory.
In a nutshell, Valence Bond Theory and hybridization would be really useful in the world of chemistry since they explain how different molecules and compounds bond. The principles forming the backbone of these theories help scientists unravel the finer nuances in chemical structures and predict properties with greater accuracy. While we continue to explore the applications and implications of Valence Bond Theory and hybridization, we come to understand and appreciate more in this interesting world of chemical bonding and lay foundations for much more excitement in the realm of chemistry.
FAQ
Difference between VBT (Valence bond theory) and MOT (molecular orbital theory)?
How does hybridization help stabilize a molecule?
Can Valence Bond Theory (VBT) And MOT predict the geometry of all molecules?
There are several practical applications of Valence Bond Theory and hybridization.
Post By Sudhir Nama Sir
0 Comments