Classify bonding, antibonding and nonbonding orbitals

Bonding character is decided by what the orbital does to electron density and energy, not by whether the orbital happens to contain electrons.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Classify bonding, antibonding and nonbonding orbitals

Look at density between the nuclei

Bonding character is decided by what the orbital does to electron density and energy, not by whether the orbital happens to contain electrons.

Compare the phase of overlapping lobes and check for a node between the nuclei. These features separate bonding, antibonding and nonbonding molecular orbitals.

Three kinds of molecular orbital

A bonding molecular orbital results from constructive, in-phase overlap. It has increased electron density in the internuclear region and lies lower in energy than the parent atomic orbitals.

An antibonding molecular orbital results from destructive, out-of-phase overlap. It has a node between the nuclei, lies higher in energy than the parent orbitals, and is marked with an asterisk, such as σ* or π*.

A nonbonding molecular orbital has little or no net bonding interaction. Its energy is close to that of the parent atomic orbital and its electron density is often localised mainly on one atom.

Why phase changes energy

In-phase combination reinforces the wavefunction between the nuclei. Electrons in this region are attracted to both nuclei and help hold them together, so the bonding orbital is stabilised.

Out-of-phase combination cancels the wavefunction between the nuclei, creating an internuclear node. Electron density is removed from the region that would hold the nuclei together, so the antibonding orbital is destabilised.

A nonbonding orbital is not simply an empty orbital. It is an orbital whose shape, symmetry or energy prevents a significant stabilising or destabilising interaction with the other orbital involved.

Worked example: combining two 1s orbitals

For 1s + 1s in phase, the lobes have the same sign and produce σ(1s). Density builds between the nuclei and the level falls below the H 1s atomic levels.

For 1s − 1s, the signs oppose and produce σ*(1s). A node appears midway between the nuclei and the level rises above the parent levels.

Try this

Classify σ(1s) and σ*(1s), giving one shape clue and one energy clue for each.

Check your answer

σ(1s) is bonding: it has internuclear density and lower energy. σ*(1s) is antibonding: it has an internuclear node and higher energy.

Practise with a lone pair

Suppose an oxygen lone-pair orbital has unsuitable symmetry to overlap with the neighbouring framework. Its shape and energy change very little.

Describe it as nonbonding because it makes little net contribution to bonding, not merely because you recognise it as a lone pair.

Try this

An orbital remains mainly on O and has almost the same energy as the O parent orbital. How should it be classified, and why?

Check your answer

It is nonbonding because there is little effective overlap and little stabilisation or destabilisation relative to the parent orbital.

Apply all three tests

For any unfamiliar orbital, inspect phase, internuclear density and relative energy together. One clue should support the others.

Keep bonding character separate from occupancy: an unoccupied bonding orbital is still bonding, and an occupied antibonding orbital is still antibonding.

Try this

A higher-energy orbital has opposite phases on adjacent atoms and a node between them. Classify it even if it is empty.

Check your answer

It is antibonding. The opposite phases, internuclear node and raised energy decide its character; occupancy does not.

Common mistake: using occupancy as the label

‘Occupied’ and ‘unoccupied’ describe electron filling. ‘Bonding’, ‘antibonding’ and ‘nonbonding’ describe the orbital itself.

Do not call every lone-pair orbital nonbonding automatically. In a conjugated system, a lone pair may overlap and contribute to a delocalised bonding or antibonding molecular orbital.

Try this

Correct: ‘The LUMO is antibonding because all empty orbitals are antibonding.’

Check your answer

A LUMO may be bonding, antibonding or nonbonding. Its phase, nodes and energy determine its character; LUMO only means lowest unoccupied molecular orbital.

Check your understanding

A complete classification names the orbital, describes its internuclear density or node, and relates its energy to the parent atomic orbitals.

Next, classify molecular orbitals by rotational symmetry as σ or π without confusing that label with bonding character.

Try this

State the decisive difference between bonding and antibonding combinations.

Check your answer

Bonding combinations build electron density between nuclei and lower energy; antibonding combinations create an internuclear node and raise energy.