antibondingorbitals

Antibonding orbitals are molecular orbitals that arise from the destructive combination of atomic orbitals. They have a node, or region of zero electron density, between the nuclei and are higher in energy than the original atomic orbitals that combine to form them. Antibonding orbitals are typically denoted with a star, as in sigma* (σ*) and pi* (π*), indicating their opposite phase relative to bonding orbitals.

In an antibonding orbital, the signs of the atomic orbitals cancel in the internuclear region, reducing electron density between the nuclei. This creates repulsion between the nuclei and weakens or offsets the attraction that holds the atoms together. Consequently, antibonding orbitals lie at higher energies in the molecular orbital diagram. The overall bond strength is determined by the balance between bonding and antibonding electrons.

Electron occupancy in antibonding orbitals affects bond order, defined as (number of bonding electrons minus number of antibonding electrons) divided by two. Each electron in an antibonding orbital lowers the bond order, often weakening or even preventing bond formation if antibonding occupancy is high.

Examples include the hydrogen molecule H2, where the ground state places both electrons in the bonding sigma (σ) orbital and leaves the antibonding sigma* (σ*) orbital empty. In other molecules, antibonding orbitals may be occupied to varying degrees, influencing bond lengths, strengths, and reactivity. Antibonding orbitals also play a role in electronic transitions, with excitations from bonding to antibonding orbitals contributing to UV–visible spectra.

Overall, antibonding orbitals are a key component of molecular orbital theory, explaining why certain electron configurations weaken bonds and how electronic structure governs molecular stability.