Difference Between Valence Bond Theory And Molecular Orbital Theory

A molecular orbital is an orbital or wave function of a molecule’s electron. The function may be used to calculate the probability of finding an electron within a specified space or to predict the molecule’s chemical and physical properties. Robert Mulliken introduced the term “orbital” in 1932 to describe a one-electron orbital wave function.

Electrons around a molecule can be associated with more than one atom and are often expressed as a combination of atomic orbitals. Atomic orbitals within a molecule can interact if they have compatible symmetries. The number of molecular orbitals is equal to the number of atomic orbitals combined to form a molecule.

Molecular orbitals can generally be expressed through a linear combination of atomic orbitals (abbreviated to LCAO). These LCAOs are useful in the estimation of the formation of these orbitals in the bonding between the atoms that make up a molecule.

The Schrodinger equation used to describe the electron behavior for molecular orbitals can be written in a method similar to that for atomic orbitals.

It is an approximate method for representing molecular orbitals. It’s more of a superimposition method where constructive interference of two atomic wave function produces a bonding molecular orbital whereas destructive interference produces non-bonding molecular orbital.

What Is Molecular Orbital Theory (MOT)?

Molecular orbital theory describes the distribution of electrons in molecules in much the same way that the distribution of electrons in atoms is described using atomic orbitals. Using quantum mechanics, the behavior of an electron in a molecule is still described by a wave function, Ψ, analogous to the behavior in an atom. Just like electrons around isolated atoms, electrons around atoms in molecules are limited to discrete (quantized) energies. The region of space in which a valence electron in a molecule is likely to be found is called a molecular orbital (Ψ2). Like an atomic orbital, a molecular orbital is full when it contains two electrons with opposite spin.

Facts About Molecular Orbital Theory (MOT)

The molecular orbital theory explains the chemical bonding of molecule using hypothetical molecular orbitals.
Molecular orbital theory was first proposed by F. Hund and R.S Mulliken in 1932.
The molecular orbital theory does not define anything about hybridization of orbitals.
Molecular orbital theory can be applied on polyatomic molecules.
Molecular orbital theory explains about the mixing of atomic orbitals when forming molecules.
In molecular orbital theory, atomic orbitals which form molecular orbitals, do not retain their individual characteristic nature.
In MOT, bonds are localized to both two atoms and molecules.
MOT is somehow complex and tedious especially in terms of calculations.
In molecular orbital theory, all the electrons of the valence shell are represented as having taken part in the bonding.
In molecular orbitals theory, formation of the molecular orbitals is based on the Linear Combination of atomic orbitals (LCAO) approximation method, whereby each molecular orbital is constructed from a superposition of atomic orbitals belonging to the atoms in the molecule.
In molecular orbital theory, there is an elaborate explanation of paramagnetic character of oxygen.
In molecular orbital theory, resonance does not play any role.

What Is Valence Bond Theory (VBT)?

Valence bond (VB) theory is a chemical bonding theory that explains the chemical bonding between two atoms. Like molecular orbital (MO) theory, it explains bonding using principles of quantum mechanics. According to valence bond theory, bonding is caused by the overlap of half-filled atomic orbitals. The two atoms share each other’s unpaired electron to form a filled orbital to form a hybrid orbital and bond together. Sigma and pi bonds are part of valence bond theory.

Valence bond theory predicts covalent bond formation between atoms when they have half-filled valence atomic orbitals, each containing a single unpaired electron. These atomic orbitals overlap, so electrons have the highest probability of being within the bond region. Both atoms then share the single unpaired electrons to form weakly coupled orbitals.

Facts About Valence Bond Theory (VBT)

Valence bond theory is a basic theory that is used to explain the chemical bonding of atoms in a molecule.
Valence Bond theory was first proposed by W.Heitler and F.London in 1927.
Valence bond theory defines the hybridization of molecular orbitals.
Valence bond theory can only be applied for diatomic molecules.
Valence bond theory explains about molecules occupying atomic orbitals. It explains the bonding of atomic orbitals.
In valence bond theory, atoms which are involved in the bond formation, maintain their individual characteristic nature.
In VBT, bonds are localized to two atoms and not molecules.
VBT has simplicity and convenience especially in terms of calculations.
In valence bond theory, some of the valence electrons are represented as not shared and not involved in the formation of the molecule.
In valence bond theory, the resulting molecular orbital is obtained by the combination of two wave functions of two unpaired electrons.
In valence bond theory, there is no explanation of paramagnetic nature of oxygen.
In valence bond theory, resonance plays an important role.