Distinguish atomic and molecular orbitals

At H₂ level, you used atomic orbitals to describe where an electron is likely to be found around one nucleus. That idea remains useful, but it is not enough once atoms interact to form a molecule.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Distinguish atomic and molecular orbitals

Start from atomic orbitals

At H₂ level, you used atomic orbitals to describe where an electron is likely to be found around one nucleus. That idea remains useful, but it is not enough once atoms interact to form a molecule.

In molecular-orbital theory, compatible atomic-orbital wavefunctions combine. The new molecular orbitals belong to the whole molecule and may extend across two or more nuclei.

Atomic orbitals and molecular orbitals

An atomic orbital, such as H 1s or C 2p, is centred on one atom. A molecular orbital, such as σ(1s), π(2p) or π*(2p), describes an allowed electron distribution over a molecule.

The number of molecular orbitals formed equals the number of atomic orbitals combined. Two H 1s orbitals therefore give two molecular orbitals: one bonding σ(1s) and one antibonding σ*(1s).

How the description changes when atoms combine

Think of an orbital as a wavefunction, not as a tiny path or container. When two compatible atomic orbitals overlap, their wavefunctions can combine in phase or out of phase. These combinations produce different electron-density patterns and different energies.

An electron placed in a molecular orbital is not copied into two atomic orbitals and does not have to be assigned permanently to one atom. The molecular orbital is one state, even when its electron density is spread across several atoms.

In a heteronuclear molecule, one atom may contribute more strongly to a molecular orbital than another. Unequal contributions do not turn it back into an atomic orbital: its spatial extent and energy still describe the molecule as a whole.

Worked example: H₂

Begin with one 1s atomic orbital on each H atom. Combining the two basis functions gives σ(1s), with enhanced electron density between the nuclei, and σ*(1s), with a node between the nuclei.

H₂ has two electrons, so both occupy the lower-energy σ(1s) orbital. On an energy-level diagram, the H 1s levels appear at the sides and the resulting molecular levels appear between them.

Try this

Explain why σ(1s) in H₂ is a molecular orbital even though it is formed from 1s atomic orbitals.

Check your answer

It spans both hydrogen nuclei and is an allowed state of the molecule. Its parent orbitals are atomic 1s orbitals, but the resulting σ(1s) orbital is molecular.

Practise the distinction

Ask two questions: Is the orbital centred on one nucleus or spread over the molecular framework? Is its energy shown as an atomic level or as a resulting molecular level?

Do not decide from the shell label alone. The symbol 2p names an atomic orbital; π(2p) names a molecular orbital formed mainly from 2p atomic orbitals.

Try this

Classify C 2p and the delocalised π orbital of ethene as atomic or molecular orbitals, and justify each choice.

Check your answer

C 2p is an atomic orbital centred on one carbon. Ethene's π orbital is a molecular orbital because it extends over both carbon atoms.

Try an unfamiliar case

A drawing of HF may show a bonding σ orbital with more electron density near F than near H. That uneven distribution reflects the different atomic-orbital energies and contributions.

The correct classification still depends on whether the orbital describes one atom or the bonded molecule.

Try this

A student says, ‘The bonding orbital in HF is an F atomic orbital because most of its density is near F.’ Correct the statement.

Check your answer

The orbital is a molecular orbital because it extends over the HF molecule. Greater F contribution makes it polarised towards F, not atomic.

Common mistake: treating orbitals as particles

Atomic orbitals do not collide and fuse like solid objects. Their wavefunctions are combined mathematically to form molecular wavefunctions with new shapes and energies.

Another common mistake is to draw one molecular electron on both parent atomic-orbital levels. Once the molecular levels are formed, place each electron on one molecular-orbital level.

Try this

Correct: ‘Two atomic orbitals collide, and each electron remains in its original orbital.’

Check your answer

Compatible atomic-orbital wavefunctions combine to form molecular orbitals. Electrons then occupy the resulting molecular levels.

Check your understanding

You should now be able to distinguish atomic from molecular orbitals by their spatial domain and to apply the rule that N atomic orbitals form N molecular orbitals.

Next, use the phase and electron-density pattern of a molecular orbital to decide whether it is bonding, antibonding or nonbonding.

Try this

In one sentence, distinguish an atomic orbital from a molecular orbital without using the words ‘small’ or ‘large’.

Check your answer

An atomic orbital is centred on one nucleus, whereas a molecular orbital is an allowed electron distribution over the molecule.