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 Incoming links: Explaining the geometry of simple molecules using molecular orbital energy-level diagrams built by using symmetry principles by Sérgio P. Machado and Roberto B. Faria

Ammonia molecular orbital diagram Created 2026-10-20 Updated 2026-10-31

In pyramidal Ammonia, the hydrogen symmetry orbitals span

A_{1} \oplus E

of the C3v molecular point group. Nitrogen

2 s

and

2 p_{z}

both have

A_{1}

(2 p_{x}, 2 p_{y})

span

E

. In the planar D3h molecular point group, these become

A_{1}^{'}

,

A_{2}^{''}

and

E^{'}

, respectively, while the hydrogen combinations span

A_{1}^{'} \oplus E^{'}

. Thus the planar out-of-plane lone pair is a symmetry-isolated nonbonding orbital; pyramidalization permits mixing with the other

a_{1}

functions.^[ref]

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Past exam of the chemistry A course of the University of Cambridge / 2021 / Paper 1 / A4 / b / ii / Solution Created 2026-10-20 Updated 2026-10-31

For planar Ammonia, the nitrogen valence atomic orbitals have symmetries

2 s : A_{1}^{'}

,

(2 p_{x}, 2 p_{y}) : E^{'}

and

2 p_{z} : A_{2}^{''}

. The hydrogen symmetry orbitals span

A_{1}^{'} \oplus E^{'}

. Consequently the in-plane orbitals form a bonding

a_{1}^{'}

orbital, a bonding

e^{'}

pair and their antibonding partners. The out-of-plane

2 p_{z}

has no matching hydrogen combination and remains a nonbonding orbital. With the core omitted, the conventional valence occupation is

(2 a_{1}^{'})^{2} (1 e^{'})^{4} (1 a_{2}^{''})^{2} .

The preceding ammonia molecular orbital diagram shows the correlation. On pyramidalization,

A_{1}^{'}

and

A_{2}^{''}

both correlate with

A_{1}

of the C3v point group, permitting s–p orbital mixing. The pure planar lone pair can acquire bonding character, lowering its energy and helping stabilize the pyramid. Symmetry alone does not provide a numerical inversion barrier.^[ref]

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