What are Molecular Orbitals?

The space in a molecule in which the probability of finding an electron is maximum can be calculated using the molecular orbital function. Molecular orbitals are basically mathematical functions that describe the wave nature of electrons in a given molecule.

These orbitals can be constructed via the combination of hybridized orbitals or atomic orbitals from each atom belonging to the specific molecule. Molecular orbitals provide a great model via the molecular orbital theory to demonstrate the bonding of molecules.

Types of Molecular Orbitals

According to the molecular orbital theory, there exist three primary types of molecular orbitals that are formed from the linear combination of atomic orbitals. These orbitals are detailed below.

Anti Bonding Molecular Orbitals

The electron density is concentrated behind the nuclei of the two bonding atoms in anti-bonding molecular orbitals. This results in the nuclei of the two atoms being pulled away from each other. These kinds of orbitals weaken the bond between two atoms.

Non-Bonding Molecular Orbitals

In the case of non-bonding molecular orbitals, due to a complete lack of symmetry in the compatibility of two bonding atomic orbitals, the molecular orbitals formed have no positive or negative interactions with each other. These types of orbitals do not affect the bond between the two atoms.

Formation of Molecular Orbitals

An atomic orbital is an electron wave; the waves of the two atomic orbitals may be in phase or out of phase. Suppose Ψ_A and Ψ_B represent the amplitude of the electron wave of the atomic orbitals of the two atoms A and B.

Case 1: When the two waves are in phase so that they add up and amplitude of the wave is Φ= Ψ_A + Ψ_B

Case 2: when the two waves are out of phase, the waves are subtracted from each other so that the amplitude of the new wave is Φ ´= Ψ_A – Ψ_B

Characteristics of Bonding Molecular Orbitals

• The probability of finding the electron in the internuclear region of the bonding molecular orbital is greater than that of combining atomic orbitals.
• The electrons present in the bonding molecular orbital result in the attraction between the two atoms.
• The bonding molecular orbital has lower energy as a result of attraction and hence has greater stability than that of the combining atomic orbitals.
• They are formed by the additive effect of the atomic orbitals so that the amplitude of the new wave is given by Φ= Ψ_A + Ψ_B
• They are represented by σ, π, and δ.

Characteristics of Anti-bonding Molecular Orbitals

• The probability of finding the electron in the internuclear region decreases in the anti-bonding molecular orbitals.
•  The electrons present in the anti-bonding molecular orbital result in the repulsion between the two atoms.
• The anti-bonding molecular orbitals have higher energy because of the repulsive forces and lower stability.
• They are formed by the subtractive effect of the atomic orbitals. The amplitude of the new wave is given by Φ ´= Ψ_A – Ψ_B
• They are represented by σ^∗, π^∗, δ^∗

Why are Antibonding Orbitals Higher in Energy?

The energy levels of bonding molecular orbitals are always lower than those of anti-bonding molecular orbitals. This is because the electrons in the orbital are attracted by the nuclei in the case of bonding Molecular Orbitals whereas the nuclei repel each other in the case of the anti-bonding Molecular Orbitals.