Shapes of molecules
Section: 3 Chemical Bonding | Syllabus: Cambridge AS Level Physics 9702
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. Shape of NH₄⁺ 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 Tip When 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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