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How do atomic orbitals combine to form molecular orbitals?
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covalent bond
In polar covalent bonds, such as that between hydrogen and oxygen atoms, the electrons are not transferred from one atom to the other as they are in an ionic bond. Instead, some outer electrons merely spend more time in the vicinity of the other atom. The effect of this orbital distortion is to induce regional net charges that hold the atoms together, such as in water molecules.
In molecular orbital theory, atomic orbitals combine to form molecular orbitals (MOs) through a process known as the linear combination of atomic orbitals (LCAO) approximation. In this approach, each molecular orbital is constructed as a superposition of the atomic orbitals belonging to the atoms in a molecule.1
The shape and energy of these resulting molecular orbitals depend on how the individual atomic orbitals interfere with one another:1
- Constructive interference: When atomic orbitals are summed, their amplitudes reinforce each other in the internuclear region. This creates a bonding orbital, which has an enhanced electron density between the nuclei, resulting in a lower energy state compared to the isolated atomic orbitals.2
- Destructive interference: When the difference between atomic orbitals is taken, their amplitudes subtract from one another. This creates a nodal plane—a region of zero amplitude—between the nuclei. This results in an antibonding orbital, which excludes electrons from the internuclear region and possesses a higher energy state.2
Generally, for every n atomic orbitals that combine, n molecular orbitals are formed, which are then filled by valence electrons according to the building-up principle and the Pauli exclusion principle.2