Difference Between Bonding and Antibonding Molecular Orbitals

Bonding vs Antibonding Molecular Orbitals
 

The difference between bonding and antibonding molecular orbitals can be best explained using “Molecular orbital theory.” These two types of molecular orbitals are formed when covalent chemical bonds are formed. The most significant difference between bonding and antibonding molecular orbitals is their energy levels compared to the parent atomic orbitals. This energy level difference leads to other differences between two molecular orbitals.

Bonding and antibonding molecular orbitals are formed by linear combination atomic orbitals. The following key concepts are very important, to understand the difference between bonding and antibonding molecular orbitals.

Aufbau principle – orbitals with the lowest energy are filled first.

Pauli exclusion principle – The maximum number of electrons (with opposite spins) that can occupy an orbital is two.

Hund’s rule – When there are several Molecular Orbitals with equal energy, the electrons occupy the Molecular Orbitals one at a time before two occupy the same Molecular Orbital.

What are Bonding Molecular Orbitals?

Bonding molecular orbitals are formed from atomic orbitals by in-phase combination of atomic orbitals. It increases the electron density between bonded atoms. Their energy is lower than the atomic orbitals. Electrons are first filled to the bonding molecular orbitals and they stabilize the molecule since they associate less energy than the electron in the parent atom.

Difference Between Bonding and Antibonding Molecular Orbitals

Molecular orbital diagram for Hydrogen

What are Antibonding Molecular Orbitals?

Antibonding molecular orbitals are formed by out-of-phase combination of atomic orbitals and it decreases the electron density between two atoms. In antibonding molecular orbitals, the energy is higher than the atomic orbitals which formed them. Due to this fact, when electrons are filled into antibonding molecular orbitals, it destabilizes the bond between two atoms.

 Bonding vs Antibonding Molecular Orbitals

H2 1sσ* antibonding molecular orbital

What is the difference between Bonding Molecular Orbitals and Antibonding Molecular Orbitals?

• Energy:

ENERGYAntibonding molecular orbitals > ENERGYBonding molecular orbitals

• Bonding molecular orbitals have lower energy compared to the parent atomic orbital.

• Antibonding molecular orbitals possess high energy than that of parent atomic orbitals.

• Generally, electrons are first filled into lower energy levels. Therefore, electrons are first filled to bonding molecular orbitals and then to antibonding molecular orbitals.

• Stability:

• Bonding molecular orbitals are more stable than both antibonding molecular orbitals and parent atomic orbital.

• Antibonding molecular orbitals are less stable than both bonding molecular orbitals and parent atomic orbtials.

• The main reason for the difference in stability is the energy level difference. The higher the energy is less is stability. Lower the energy is more is the stability.

• Electron Availability:

• The probability of finding an electron is very high in bonding molecular orbitals.

• Finding an electron in antibonding molecular orbitals is minimum.

• Contribution for the shape of the molecule:

• Bonding molecular orbitals directly contribute to the shape of the molecule.

• Antibonding molecular orbitals do not contribute to the shape of the molecule.

 

Images Courtesy:

  1. MO diagram Hydrogen by CCoilCC BY-SA 3.0)
  2. H2 1sσ* antibonding molecular orbital via Wikicommons (Public Domain)