Valence Bond Theory is a simple way to explain how atoms form chemical bonds in a molecule. According to this theory, a bond is formed when two atoms share their unpaired electrons by overlapping their orbitals. It helps us understand the shapes of molecules and the strength of bonds. While VBT works well for many molecules, it doesn't always explain everything, especially in more complex cases. Still, it gives a basic idea of how atoms come together to form stable compounds.

What is Valence Bond Theory?

Valance Bont Theory helps us understand how atoms form covalent bonds by overlapping their atomic orbitals. This concept was first introduced in 1927 by Hewither and London, based on quantum mechanics, later developed further by Pauling and Slater in 1931. According to this theory, a covalent bond forms when two atoms share a pair of electrons, each contributing one electron from their half-filled orbitals. These orbitals must have similar energy levels and the electrons should have opposite spin.

Chemistry Notes Free PDFs

The overlapping of orbitals allows the electron density to increase between the nuclei of the two atoms. As a result, attractive forces grow together than repulsive ones, and the system becomes more stable. This overlap leads to the formulation of a directional bond, meaning the bond forms in a specific direction based on how the orbitals align in space.

Valence Bond Theory gives a clear picture of how bonds form, why they have certain shapes and strengths, and how electrons behave during bonding. Throughout this article, we’ll explore how this theory works in more detail, including its main ideas, how it explains energy changes in bond formation, real-world importance, and where it falls short in explaining some types of bonding.

The postulates of the valence bond theory are

• A covalent bond is formed only when half-filled orbitals of two atoms overlap each other.
• Each overlapping atomic orbital should contain an unpaired electron with an opposite spin.
• The overlapping atomic orbitals must have nearly the same i.e. comparable energies.
• The electron spins are neutralized and electrons get paired up on overlapping of atomic orbitals.
• During the overlap of atomic orbitals, the electron density increases in between the two nuclei and therefore repulsion between the two nuclei of bonded atoms decreases. This results in the liberation of energy and an increase in the attractive forces between atoms.
• A bond is formed at an equilibrium distance when the system has minimum potential energy and maximum stability.
• The strength of a covalent bond depends upon the extent of overlap of orbitals between the two atoms. Greater the extent of overlap between atomic orbitals more is the energy released and stronger is the bond formed.
• The number of covalent bonds formed by an atom of an element is equal to the number of unpaired electrons present in the valence shell of the atom.
• Each atomic orbital (except-s-orbital) has a particular direction in space. Hence covalent bonds formed by the overlap of atomic orbitals possess directional characteristics.
• The geometry of a molecule is decided by the directional orientation of overlapping orbitals, as the orbital is spherically symmetrical and overlaps in any direction. But p, d, and f orbitals have particular directions in space, hence the geometry and bond angle depends upon the orientation of overlapping orbitals. To minimize electronic repulsion and to have maximum symmetry the orbitals orient in such a way that they are far apart from each other.

Read more about Chemical Bonding, here.

Interacting Forces in Covalent Bond Formation

When two atoms approach each other, forces of attraction and repulsion are developed. The interacting forces are due to-

• The nucleus of one atom attracts electrons of the other atom and vice versa.
• There is repulsion between the electrons of the two atoms (as all the electrons carry a negative charge.)
• There is repulsion between two nuclei (as both are positively charged)

If forces of attraction are stronger than forces of repulsion, the total energy of the system decreases and a covalent bond results. The greater the decrease in energy in the formation of a bond, the greater is the strength of the bond.

If forces of repulsion are stronger than forces of attraction total energy of the system increases and a covalent bond is not formed

Know more about F Block Elements, here.

Potential Energy Diagram, Formation of H2 Molecule

In order to attain stability, atoms combine to form a molecule. Stability is achieved by a decrease in the potential energy of the system of combining atoms forming a molecule. In short, the energy of a molecule is less than the sum of energies of the combining atoms.

Consider the formation of a hydrogen molecule from hydrogen atoms.

When the two hydrogen atoms are separated by a large distance, those two atoms can neither attract nor repel each other. The potential energy of the two hydrogen atoms is arbitrarily taken to be zero.

As the two hydrogen atoms containing electrons with opposite spins approach each other, attractive forces, as well as repulsive forces, operate, attractive forces being stronger than repulsive ones, the potential energy decreases.

An equilibrium inter-nuclear distance is the one, where attractive forces are balanced by repulsive forces and the potential energy of the system attains a minimum value.

At this stage, the maximum overlap of atomic orbitals takes place and a stable bond is formed. The inter-nuclear distance is 74 pm, which is the H-H bond length. The energy liberated is 436 kJ/mol.; which is known as bond energy.

If the distance between the two atoms is decreased further, repulsive forces exceed the attractive forces and the potential energy of the system shoots up.

If the atoms containing electrons with parallel spins approach each other, the potential energy of the two hydrogen atoms goes on increasing, and bond formation does not take place.

Importance of Valence Bond Theory

Valence bond theory introduced five new concepts in chemical bonding

• Delocalization of electrons over the two nuclei.
• Shielding effect of electrons.
• The essential covalent character of bond.
• The presence of partial ionic nature in a covalent bond.
• The concept of resonance and connection between resonance energy and molecular stability

Limitations of Valence bond theory

Valance Bond theory explains covalent bonds formed by sharing one electron from each atom but doesn’t fully explain cases when both electrons come from the same atom, known as coordinate covalent bonds.However, it offers no explanation for the formation of a coordinate covalent bond in which both the electrons are contributed by one of the bonded atoms.

Oxygen molecules should be diamagnetic according to this theory. The two atoms in the oxygen molecule should have closed electronic shells which would give no unpaired electrons to the molecule making it diamagnetic. However, experimentally the molecule is found to be para-magnetic having two unpaired electrons. Thus, this theory fails to explain.

Valence bond theory does not explain the bonding in electron-deficient molecules like 

B2H6

B2H6, in which the central atom possesses a fewer number of electrons paramagnetism of oxygen molecules than required for an octet of electrons.

Number of Orbitals and Types of Hybridization

Hybridisation is the process where atomic orbitals combine to form new, equivalent orbitals that help explain the shape and bonding in molecules. 

The orbitals of metal and ligand when energetically comparable then only undergo hybridization. Hybridization gives the modified orbital which is ready for bonding and creates strong bonds between the two,i.e., metal and ligand. The (n-1)d, ns, np, or ns, np, and orbitals from Metal and ligand take part in hybridisation and after that the so generated hybridized orbitals occupied various orientation in three dimensional space, more precisely the hybridized orbital follows a specific geometry according to the type of their parent orbital which take part in mixing.

Applications of Valence Bond Theory

The applications of valence bond theory is noticeable as it is able to change the visualization of chemists. Few major applications are given below:

• Valence bond theory opens up a new avenue to explain the root of covalent bond formation. This theory says that the covalent bond is a result of mixing of two atomic orbitals which further change the concept of sharing electrons mentioned in ancient chemistry literature.
• The nature of covalent bond is transparently explained through the concept of valence bond theory. According to the theory the sustainability of the covalent bond is indeed dependent on the nature of overlapping.
• The formation of HF molecules is described by the valence bond theory in which the mixing of hydrogen 1s and 2p orbital of fluorine generates the H-F bond. 

Modern Approaches of Valence Bond Theory

The modern approaches of valence bond theory are promising and complicated. To get a better understanding first of all we say about atoms and molecules. Atoms are the building blocks of molecules. That means when an atom combines a molecule generates. The identical concept is applied to explain the modern approach of valence bond theory, in which it is considered that the mixing of two atomic orbital generates molecular orbital. Therefore the bonding between two atoms is a contribution of molecular orbitals. If two atomic orbitals are combined there will be two molecular orbitals in which one has high energy and another resides at lower energy level. Through this concept the magnetic property of a molecule is also well explained Moreover, the aromaticity which was explained earlier with the help of resonance concept can be reframed by applying delocalization of electrons using molecular orbitals.

Solved Examples of Valence Bond Theory

What kind of overlapping is present in ethylene molecules?

A:The formula of ethylene is C2H4 where the both C atoms have three bonding pairs and no lone pairs, meaning they are both sp2 hybridized. As sp2 means there are 3 orbitals which are created by mixing of one s and 2p orbitals of carbons. Here one sigma bond is formed by head-on mixing of a set of sp3 orbitals of carbons and the other two sets of orbitals are involved in producing 2 pi bonds with hydrogens.

What are Coordination Compounds?

Coordination compounds are formed by metal-ligand interactions. If we focus on their molecular structure it is noticeable that a central atom is surrounded by ligands (another atom). The bonding between them is covalent in nature but if the bonding is controlled solely by the ligand’s electron pair then the covalent bond is popularly known as the dative type.

Difference between Valence Bond Theory and Molecular Orbital Theory

Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT) are two ways to understand how atoms bond together. VBT explains bonding as the overlap of atomic orbitals between two atoms, like a handshake between them. On the other hand, MOT sees bonding as the mixing of atomic orbitals to form new molecular orbitals that spread across the whole molecule. Both help us understand how molecules form and behave, just from different angles.

Learn more about S Block Elements, here.

Also, check out the other topics of Chemistry, here.

Ace your exams and level up your preparation game with Testbook's curated study materials and exam practice sets. Download the Testbook App for free today to take advantage of some exclusive offers now.