Why does the energy ordering of the molecular orbitals of the period 2 diatomic molecules change

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Why does the energy ordering of the molecular orbitals of...

Key Concepts

Effect of Nuclear Charge and Orbital Energies

The atomic orbital energies are influenced by the effective nuclear charge. As one moves across the period, increasing nuclear charge lowers the energy of the orbitals, reducing the energy gap between s and p orbitals and altering the degree to which orbitals can mix. This adjustment in orbital energies and mixing strength leads to observable changes in the ordering of molecular orbitals across different diatomic molecules.

Molecular Orbital Theory

This theory explains how atomic orbitals combine to form molecular orbitals that are delocalized over the entire molecule. The resulting molecular orbitals come in bonding and antibonding varieties. Their relative energies and occupancies determine many of the chemical and physical properties of the molecule, such as bond order, magnetism, and stability.

Orbital Energy Ordering

Molecular orbitals are arranged in a specific order of energy based on the combination of atomic orbitals. Changes in factors such as the relative energies of the contributing atomic orbitals, and the extent of overlap between them, can lead to different energy orderings in different molecular systems. As conditions change, such as an increase in nuclear charge, the ordering between bonding and antibonding orbitals can shift.

s-p Orbital Mixing

In period 2 diatomic molecules, the energy proximity of the 2s and 2p atomic orbitals allows for mixing between these orbitals (s-p mixing). This mixing alters the energies of the resulting molecular orbitals. As the nuclear charge increases, the energies of the atomic orbitals change, which in turn modifies the extent of s-p mixing and thereby the energy ordering of the molecular orbitals.

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