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:


symbolmeaningimage example
Acentral atom
Xsurrounding atoms (bonding pairs)
Elone‑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  
2AX₂Linear180°HC≡CH (terminal C)acetylene axe
3AX₃Trigonal
 planar
120°CH₂Oformaldehyde axe
4AX₄Tetrahedral109.5°CH₄methane axe
5AX₅Trigonal 
bipyramidal
90° / 120°PF₅phosphorus pentafluoride axe
6AX₆Octahedral90°SF₆sulfur hexafluoride axe



3. Molecular Shapes with Lone Pairs


 SN  AXE   molecular
shape
 
 approx. angle  example  Image  
3AX₂EBent (V‑shaped)118°SO₂so2 axe
4AX₃ETrigonal pyramidal107°NH₃nh3 axe
4AX₂E₂Bent104.5°H₂Oh2o axe
5AX₄ESeesaw90° / 117°SF₄sf4axe
5AX₃E₂T‑shaped90°ClF₃clf3_axe
5AX₂E₃Linear180°XeF₂xef2 axe
6AX₅ESquare pyramidal90°BrF₅brf5 axe
6AX₄E₂Square planar90°XeF₄xef4 axe

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.


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°.