Bonding
Molecular Geometry
3D shape of molecules — VSEPR predicts angles from electron pair repulsion
Molecular geometry is the 3D arrangement of atoms in a molecule. Predicted by VSEPR theory (Valence Shell Electron Pair Repulsion): electron pairs around central atom repel each other, arranging to maximize separation. Common geometries: linear (180°, CO₂), trigonal planar (120°, BF₃), tetrahedral (109.5°, CH₄), trigonal pyramidal (107°, NH₃), bent (104.5°, H₂O), trigonal bipyramidal (PCl₅), octahedral (SF₆). Lone pairs occupy more space than bonding pairs — distort angles. Geometry determines polarity, reactivity, biology.
- VSEPR theoryValence Shell Electron Pair Repulsion
- Tetrahedral angle109.5° (e.g., CH₄)
- Trigonal planar120° (e.g., BF₃)
- Linear180° (e.g., CO₂)
- Bent (water)104.5° (lone pair compression)
- Octahedral90° angles (e.g., SF₆)
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Why geometry matters
- Polarity. Determines whether molecule is polar.
- Reactivity. Steric effects from shape.
- Biology. Enzyme-substrate fit, drug-receptor binding.
- Spectroscopy. IR active modes depend on geometry.
- Crystal packing. Solid-state arrangement.
- Solubility. Depends on shape and polarity.
- Material design. Polymer chains, liquid crystals.
Common misconceptions
- Lone pairs don't affect shape. Critical for accurate prediction.
- Geometry is just 2D Lewis structure. 3D structure differs.
- Tetrahedral always 109.5°. Lone pairs compress.
- Polar bonds = polar molecule. Symmetry can cancel.
- Double bond = 2 domains. One domain.
- Geometry doesn't affect biology. Critical for binding.
Frequently asked questions
What's VSEPR theory?
Valence Shell Electron Pair Repulsion. Electron pairs (bonds + lone pairs) around central atom arrange to maximize distance from each other. Predicts geometry from total electron domains: 2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral. Each has variations based on lone pairs.
How do lone pairs affect geometry?
Lone pairs occupy more space than bonding pairs (held by only one atom, more diffuse). Compress bond angles. CH₄: tetrahedral 109.5°. NH₃: trigonal pyramidal 107° (one lone pair). H₂O: bent 104.5° (two lone pairs). Each additional lone pair compresses angle by ~2.5°.
How do you predict geometry?
Steps. (1) Draw Lewis structure. (2) Count electron domains around central atom (each bond — single, double, triple — = 1 domain; each lone pair = 1). (3) Determine electron geometry from total domains. (4) Determine molecular geometry from atomic positions only (lone pairs invisible). For example, NH₃: 4 domains → tetrahedral electron geometry → trigonal pyramidal molecular.
What's the difference between electron and molecular geometry?
Electron geometry: arrangement of all electron domains (bonds + lone pairs). Molecular geometry: arrangement of atoms only. Identical when no lone pairs (CH₄, BF₃). Different when lone pairs (NH₃: tetrahedral electrons, trigonal pyramidal atoms). Bond angles compressed by lone pairs.
What about expanded geometries?
Period 3+ central atoms can have 5 or 6 electron domains. PCl₅: 5 domains, trigonal bipyramidal (3 equatorial + 2 axial). SF₆: 6 domains, octahedral. With lone pairs: SF₄ (seesaw), ClF₃ (T-shaped), XeF₄ (square planar). Lone pairs preferentially occupy equatorial positions.
How does geometry affect polarity?
Symmetric polar bonds can cancel — molecule nonpolar. CO₂: linear; opposite C=O bonds cancel; nonpolar. H₂O: bent; bonds don't cancel; polar. CCl₄: tetrahedral, nonpolar (symmetric). CHCl₃: tetrahedral but asymmetric, polar. Vector sum of bond dipoles gives net molecular dipole.
What about double bonds?
Double or triple bonds count as one domain (region of electron density). Only the σ-framework determines geometry. CO₂: 2 domains → linear. C₂H₄ (ethene): 3 domains around each C → trigonal planar (sp² hybridized). Ethyne (C₂H₂): 2 domains around each C → linear (sp hybridized).