How does molecular-orbital theory describe electrons in a covalent bond?

Molecular-orbital (MO) theory describes electrons in a covalent bond not as localized particles between two atoms, but as being distributed over all the nuclei within a molecule. This spatial distribution is defined by a molecular orbital, which is a wave function derived from the combination of atomic orbitals belonging to the constituent atoms.12

Key aspects of this description include:

• Formation: When atoms approach, their atomic orbitals combine to form an equal number of molecular orbitals. These combinations occur through constructive or destructive interference.231
• Bonding vs. Antibonding:
  • Bonding orbitals result from constructive interference, creating enhanced electron density between nuclei. These are at a lower energy level than the original atomic orbitals.23
  • Antibonding orbitals result from destructive interference, creating a nodal plane (a region of zero amplitude) between nuclei. These are at a higher energy level.23
• Electron Occupation: Electrons fill these orbitals according to the Pauli exclusion principle and the building-up principle, typically occupying the lower-energy bonding orbitals to stabilize the molecule.3
• Delocalization: Unlike models that treat bonds as localized electron pairs, MO theory emphasizes that electrons can be delocalized across multiple atoms, which effectively explains phenomena like hypervalence and electron-deficient compounds.45