Molecular Orbital Theory for Homonuclear Diatomics
A shared pair between two atoms is a picture that fails for oxygen, which is magnetic. Combining atomic orbitals into orbitals that belong to the whole molecule gives a scheme that predicts bond order, magnetism and ionisation together.
What a learner can do afterwards
- Combines two atomic orbitals into a bonding and an antibonding combination and explains the energy difference
- Fills a diagram for a second-row diatomic, including the s and p mixing that reorders the levels before oxygen
- Calculates bond order and links it to measured bond length and dissociation energy
- Uses the diagram to predict that oxygen is paramagnetic and explains why the shared-pair picture cannot
1 · Read
Molecular orbital theory builds molecules the way atomic theory builds atoms. Two atomic orbitals combine into two molecular orbitals: a bonding one where the waves add between the nuclei and sit lower, and an antibonding one where they cancel and sit higher. The climb above outweighs the drop below, so filling both gives no net gain. Fill the ladder with paired spins, just like atomic orbitals.
Second row diatomics hide a surprise. Up to nitrogen, the 2s and 2p levels sit close enough to mix, which pushes the sigma orbital from the pz pair above the two pi orbitals. From oxygen onward the gap widens, mixing fades, and the normal order returns. Work boron, carbon, and nitrogen with the mixed order and oxygen and fluorine with the unmixed one. Deep down, the filled 1s pair cancels and contributes nothing.
Read bond order as half the bonding count minus the antibonding count. That single number predicts the lab facts: higher order means shorter, stronger bonds with larger dissociation energies. Nitrogen with its triple bond is short and stubborn, while fluorine with its single bond is long and weak. Count valence electrons, deal them into the ladder, and read the number off.
Magnetism is the sharpest test. Liquid oxygen sticks to a magnet because each molecule carries two unpaired electrons, which the simple shared pair picture cannot explain at all. Whenever Lewis and the orbital diagram disagree about unpaired electrons, trust the diagram. Experiment sides with it every time.
Combine, fill with the right order, halve the difference, and trust the magnet.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.