VSEPR Theory and Molecular Geometry

Valence‑Shell Electron‑Pair Repulsion (**VSEPR**) theory predicts the three‑dimensional shapes of molecules by treating regions of electron density—single bonds, multiple bonds, lone pairs, and single unpaired electrons—as point charges that repel one another. By counting these domains and applying simple rules, you can assign a geometry to virtually any main‑group molecule or polyatomic ion.
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1. Electron‑Domain Counting and AXE Notation

1. Draw a valid Lewis structure (complete octets, reasonable formal charges).
2. Count electron domains around the central atom.
3. Assign the steric number (SN) = σ bonds + lone pairs.
4. Match SN to an electron‑domain (ED) geometry.
5. Remove lone‑pair “phantoms” to get the observable molecular shape; adjust ideal angles for lone‑pair or multiple‑bond repulsion.

Chemists summarise shapes with AXE:

symbol	meaning	image example
A	central atom
X	surrounding atoms (bonding pairs)
E	lone‑pair domains on A

The steric number is X + E.

2. Electron‑Domain Geometries (no lone pairs)

SN	AXE	electron‑domain geometry	ideal angle	representative example	image
2	AX₂	Linear	180°	HC≡CH (terminal C)
3	AX₃	Trigonal  planar	120°	CH₂O
4	AX₄	Tetrahedral	109.5°	CH₄
5	AX₅	Trigonal  bipyramidal	90° / 120°	PF₅
6	AX₆	Octahedral	90°	SF₆

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3. Molecular Shapes with Lone Pairs

SN	AXE	molecular shape	approx. angle	example	Image
3	AX₂E	Bent (V‑shaped)	118°	SO₂
4	AX₃E	Trigonal pyramidal	107°	NH₃
4	AX₂E₂	Bent	104.5°	H₂O
5	AX₄E	Seesaw	90° / 117°	SF₄
5	AX₃E₂	T‑shaped	90°	ClF₃
5	AX₂E₃	Linear	180°	XeF₂
6	AX₅E	Square pyramidal	90°	BrF₅
6	AX₄E₂	Square planar	90°	XeF₄

Lone‑pair compression explains why the H–O–H angle in water (104.5°) is smaller than the ideal tetrahedral 109.5°.

4. Worked Examples

Formaldehyde (CH₂O)

Lewis structure: O═C‑H₂
SN = 3 → AX₃ → ED geometry trigonal planar → molecular shape trigonal planar.
Predicted angle ≈ 120°; observed split 121°/118° due to π‑bond localisation.

Ammonia (NH₃)

SN = 4 → AX₃E → ED geometry tetrahedral → trigonal pyramidal shape.
Lone‑pair repulsion lowers H–N–H from 109.5° to ~107°.

Terminal Carbon of Propyne (CH₃–C≡CH)

SN = 2 → AX₂ → linear; H–C–C = 180°.

5. Deviations and Limitations

• Multiple bonds count as one domain but repel slightly more than single bonds → minor angle expansion adjacent to C=C or C≡C.
• Electronegative substituents (e.g., CF₃) draw electron density, contracting nearby bond angles.
• VSEPR is qualitative; hypervalent species and transition‑metal complexes require MO or DFT methods for accurate geometries.
  

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6. Practice Problems

1. Assign the AXE formula, ED geometry, and molecular shape of sulfur dioxide, SO₂.
2. Predict the H–C–C bond angle in allene, H₂C=C=CH₂, and explain.

solutions

1. SO₂: AX₂E → ED geometry trigonal planar → bent (~118°).
2. Central carbon is sp‑hybridised (SN = 2) → linear → 180°.

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