Valence bond theory (VBT) describes the formation of covalent bonds and the electronic structure of molecules. It assumes that electrons occupy atomic orbitals of individual atoms within a molecule, and that the electrons of one atom are attracted to the nucleus of another atom.
VBT states that the overlap of incompletely filled atomic orbitals leads to the formation of a chemical bond between two atoms. The unpaired electrons are shared and a hybrid orbital is formed. VBT views bonds as weakly coupled orbitals (small overlap). When the orbitals overlap along an axis containing the nuclei, they form a σ bond. VBT accounts for the directional nature of covalent bonds.
In this article, we will learn about Valance Bond Theory, Valance Bond Theory Class 12, its various postulates, limitations, and other details. Before learning about VBT (Valance Bond Theory), let’s first learn about Hybridization.
What is Valence Bond Theory?
Valance Bond Theory also called VBT is the basic theory used to explain the structure and the bonding in coordination compounds. This theory is used to explain the formation of the bonds in the various atomic orbitals of the coordination compounds.
Valance bond theory assumes that the bonds are formed by individual atoms by the donations of electrons. But this assumption is wrong as in reality the bonds in the coordination bond are formed by the delocalization of the electrons in the orbitals of the coordination compounds. Valance Bond Theory class 11 is the basic theory that helps us visualize how various bonds are formed in nature.
What is Hybridization?
In the year 1931, scientist Linus Pauling proposed the innovative concept of hybridization. He called the process hybridization and characterized it as the shifting of the energy of particular atoms’ orbitals to produce new orbitals of equivalent energy. New orbitals, known as hybrid orbitals, emerge as a result of this process. The hybrid orbitals are shown in the image added below,
Various types of hybridization are,
One s and one p-orbital are combined together to generate two sp–hybrid orbitals with a linear structure and a bond angle of 180 degrees. For example, when BeCl2 is formed, the first atom is in the excited state 2s1 2p1, which is then hybridized to generate two sp–hybrid orbitals. BeCl2 is formed when these hybrid orbitals collide with the two p-orbitals of two chlorine atoms.
One s and one p-orbital are combined together to generate three sp2– hybrid orbitals with a planar triangular shape and a bond angle of 120 degrees.
One s and three p-orbitals are merged in this hybridization to generate four sp3– hybrid orbitals with a tetrahedral structure and a bond angle of 109 degrees 28′, or 109.5 degrees.
Number of Orbitals and Types of Hybridization
VBT explains that we have (n-1)d, ns, np, or ns, np, nd that are used for hybridization and they form hybrid orbital in which the electrons then reside. The various types of hybrid orbitals are responsible for various shapes that are shown in the table below,
Types of Hybridisation
Shape of Hybrid Orbital
History of Valence Bond Theory
Valence Bond Theory class 11 (VBT) was proposed by German physicists Walter Heinrich Heitler and Fritz Wolfgang London because, Lewis approach to chemical bonding failed to shed light on chemical bond formation. Furthermore, the valence shell electron pair repulsion theory (or VSEPR theory) had only a few applications (and also failed in predicting the geometry corresponding to complex molecules).
Schrodinger wave equation was also used to explain how a covalent bond between two hydrogen atoms formed. Valance Bond Theory focuses on the concepts of electronic configuration, atomic orbitals (and their overlapping), and atomic orbital hybridization. Chemical bonds are formed by the overlapping of atomic orbitals, with electrons localised in the bond region. The valence bond theory also explains the electronic structure of molecules formed by the overlapping of atomic orbitals. It also emphasizes the fact that the nucleus of one atom in a molecule is drawn to the electrons of the other atoms.
What is Need for Valence Bond Theory?
Lewis’ theory explained the structure of molecules. It did not, however, explain the formation of chemical bonds. VSEPR theory, on the other hand, explained the shape of simple molecules. However, it had a very limited application. It also failed to explain the geometrical properties of complex molecules. As a result, scientists were forced to develop the theory of valence bonds in order to address and overcome these limitations.
Postulates of Valence Bond Theory
Various postulates of VBT or Valance Bond Theory are,
- When two or more valance orbitals (half-filled) overlap each other they formed balanced orbital and then the electrons are filled in those orbitals accordingly.
- Only unpaired electrons of the orbital participate in the filling of valance orbital. Paired electrons do not participate in the filling of valance orbital.
- Covalent chemical bond formed by the valance atomic orbital are directional in nature.
- Valance Bond formed are Sigma Bond and Pi Bond, if orbitals overlap head to head they form Sigma Bond, if the orbital overlap sidewise they form Pi bond.
Applications of Valence Bond Theory
Various applications of VBT or Valance Bond Theory class 11 and Valance Bond Theory class 12 are,
- Valence Bond Theory explain the formation of covalent bond in various compounds.
- The strength of various bonds of the compounds are explained by Valance Bond Theory (VBT), i.e. the strength of H2 and F2 are explain using Valance Bond Theory.
Limitations of Valence Bond Theory
The limitations of the Valance Bond Theory are,
- Tetravalency of the carbon atom is not explained by the Valance Bond Theory (VBT)
- Strong Ligand and Weak Ligand are not differentiate using Valance Bond Theory (VBT)
- The localization of electrons are not explained using valance Bond Theory (VBT)
- This theory fails to explain the colour exhibited by the coordination compounds.
Modern Approach of VBT (Valance Bond Theory)
Modern Valance Bond Theory also called the Molecular Orbital Theory (MOT) is a theory that explains the bond formation of various molecule not using hybrid orbitals but by using molecular orbitals. MOT is used to much easily explains the formations of various compounds. MOT also explains the aromatic properties of various compounds.
Valence Bond Theory Examples
Valence bond theory (VBT) is used to explain how covalent bonds form in many compounds. Here are some examples of VBT:
- Water (H2O): Water is made up of two hydrogen atoms bound to an oxygen atom. Each hydrogen atom has an s-orbital with one lone electron, and oxygen has an s-orbital with an electron pair.
- Fluorine Molecule (F2): The F-F bond in the diatomic fluorine molecule is formed by the intersection of the two F atoms’ pz orbitals, each of which contains an unpaired electron.
- Ethylene (C2H4): Both C atoms have three bonding pairs and no lone pairs, meaning they are both sp2 hybridized.
- Methylamine (CH3NH2): Both the carbon and the nitrogen atom are sp3-hybridized. The C-N sigma bond is an overlap between two sp3 orbitals
VBT can also explain the difference in the length and strength of the chemical bonds in H2 and F2 molecules.
FAQs on Valance Bond Theory
Define Valence Bond Theory.
Valance Bond Theory class 11 is a theory that explains chemical bonding. According to VBT, the overlap of partially filled atomic orbitals results in the formation of a chemical bond between two atoms. The unpaired electrons are shared, resulting in the formation of a hybrid orbital.
What are Merits of Valence Bond Theory?
VBT is used to explain how covalent bonds are formed in the molecules. It is used to explain the strength of H2, F2, etc.
How are Sigma and Pi Bonds formed?
- Sigma bonds are formed when the atomic orbitals involved in the bond overlap head-to-head.
- Pi bonds, on the other hand, involve the atomic orbitals overlapping in parallel.
Who Discovered Valence Bond Theory(VBT)?
American scientists Linus Pauling and John C. Slater are credited for the discovery of Valence Bond Theory.
What are Coordination Compound?
Coordination Compounds are the compound that are formed by metal-ligand bonds. Some examples of coordination compounds are, [Fe(H2O)6]3+, [Pt(NH3)2Cl2], etc.
What are Limitations of Valance Bond Theory(VBT)?
Various limitations of the Valance Bond Theory(VBT) are,
- Valance Bond Theory (VBT) is not able to explain the formation of coordinate covalent bond.
- Valance Bond Theory (VBT) is unable to distinct between weak and strong ligands, etc.
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