Shapes of molecules
Section: 3 Chemical Bonding | Syllabus: Cambridge AS Level Chemistry (9701)
VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on the repulsion between electron pairs around the central atom.
- Electron pairs (both bonding pairs and lone pairs) repel each other and arrange themselves as far apart as possible.
- Lone pairs repel more strongly than bonding pairs, because lone pairs are closer to the central nucleus and occupy more space.
- Order of repulsion (strongest → weakest): lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.
- Multiple bonds (double, triple) count as one electron domain for VSEPR purposes.
Molecular Shapes - Required Examples
| Molecule | Bonding pairs | Lone pairs | Shape | Bond angle |
|---|---|---|---|---|
| BF₃ | 3 | 0 | Trigonal planar | 120° |
| CO₂ | 2 (double bonds) | 0 | Linear | 180° |
| CH₄ | 4 | 0 | Tetrahedral | 109.5° |
| NH₃ | 3 | 1 | Pyramidal (trigonal pyramidal) | 107° |
| H₂O | 2 | 2 | Non-linear (bent/V-shaped) | 104.5° |
| SF₆ | 6 | 0 | Octahedral | 90° |
| PF₅ | 5 | 0 | Trigonal bipyramidal | 120° (equatorial) and 90° (axial) |
Why NH₃ has a smaller angle than CH₄
- CH₄: 4 bonding pairs, 0 lone pairs → perfect tetrahedral, 109.5°.
- NH₃: 3 bonding pairs + 1 lone pair. The lone pair repels the bonding pairs more strongly → bond angle compressed to 107°.
Why H₂O has a smaller angle than NH₃
- H₂O: 2 bonding pairs + 2 lone pairs. Two lone pairs repel the bonding pairs even more → bond angle further compressed to 104.5°.
Figure 3.22: Shapes of BF₃, CO₂, CH₄, NH₃, H₂O
(Five 3D structural diagrams side by side. BF₃: flat triangle, 120°. CO₂: linear, 180°, double bonds shown. CH₄: tetrahedron, 109.5°, wedge-dash notation. NH₃: pyramid with lone pair shown on N, 107°. H₂O: V-shape with 2 lone pairs on O, 104.5°. Label all bond angles.)
Figure 3.23: Shapes of SF₆ and PF₅
(SF₆: octahedron with S at centre and 6 F atoms at 90° to each other. PF₅: trigonal bipyramid with 3 equatorial F atoms (120° apart in plane) and 2 axial F atoms (90° to equatorial). Label equatorial and axial positions and all bond angles.)
Predicting Shapes of Analogous Molecules and Ions
VSEPR can predict shapes of any molecule or ion by counting bonding pairs + lone pairs around the central atom.
- For ions, adjust the total electron count: add 1 electron per negative charge, subtract 1 per positive charge.
N has 5 valence electrons. Bonded to 4 H atoms. Charge = +1 → subtract 1 electron.
Total electron pairs = (5 + 4 − 1) ÷ 2 = 4 bonding pairs, 0 lone pairs.
Shape: tetrahedral, bond angle 109.5°.
Shape of PCl₄⁺P has 5 valence electrons. Bonded to 4 Cl. Charge = +1 → subtract 1 electron.
Total = (5 + 4 − 1) ÷ 2 = 4 bonding pairs, 0 lone pairs → tetrahedral, 109.5°.
Shape of SO₃²⁻S has 6 valence electrons. Bonded to 3 O. Charge = 2− → add 2 electrons.
Total = (6 + 6 + 2) ÷ 2 = 7 pairs. 3 bonding + 1 lone pair on S (remaining 3 are lone pairs on O, not counted on S for VSEPR). → pyramidal, <109.5°.
Exam TipWhen describing a molecular shape, name the shape of the molecule (based on atom positions only), not the arrangement of electron pairs. NH₃ is "pyramidal" - not "tetrahedral" (which would include the lone pair). Always state the bond angle.
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