What is an antibonding molecular orbital?

An antibonding molecular orbital is a type of molecular orbital that weakens or destabilizes a bond between atoms.

Here’s the simple explanation:

• When two atomic orbitals combine, they form two molecular orbitals:
  1. Bonding orbital (σ or π) → lowers energy, stabilizes the molecule.
  2. Antibonding orbital (σ or π)** → raises energy, destabilizes the molecule.
• Electrons in an antibonding orbital reduce the strength of the bond.
• It is denoted with a “*” (like σ* or π*).

Visual idea:

• Bonding orbital → electron density between the nuclei → holds them together.
• Antibonding orbital → electron density outside the nuclei → pushes them apart.

In short:
Antibonding orbital = molecular orbital that makes a bond weaker if electrons occupy it.