What determines molecular shape?
Molecular geometry is the 3D spatial arrangement of atoms in a molecule. VSEPR (Valence Shell Electron Pair Repulsion) theory predicts shapes based on the repulsion between electron domains.
Electron domains—bonding pairs and lone pairs—repel each other, positioning atoms in specific 3D shapes: linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral.
- •2 electron domains
- •0 lone pairs
- •Example: CO₂, HCN
- •180° bond angle
- •4 electron domains
- •0 lone pairs
- •Example: CH₄, CCl₄
- •109.5° bond angles
Step-by-step worked examples
Methane (CH₄) has a carbon atom bonded to 4 hydrogens with no lone pairs on carbon. What is its molecular geometry?
Count electron domains: 4 C-H bonds = 4 electron domains Count lone pairs on central atom: 0 4 domains, 0 lone pairs → tetrahedral geometry
Water (H₂O) has oxygen bonded to 2 hydrogens and has 2 lone pairs. What is its shape?
Count electron domains: 2 O-H bonds + 2 lone pairs = 4 electron domains Bent/angular shape due to repulsion pushing lone pairs Bond angle ≈ 104.5° (compressed from tetrahedral 109.5°)
Carbon dioxide (CO₂) has carbon bonded to 2 oxygens (double bonds) with no lone pairs. What geometry?
Count electron domains: 2 C=O bonds = 2 electron domains (double bond counts as one domain) 0 lone pairs 2 domains → linear geometry, 180° bond angle
Flashcards
Quick quiz
Q1.Ammonia (NH₃) has 3 N-H bonds and 1 lone pair. What is its molecular geometry?
Q2.How many electron domains does a double bond count as?
Q3.What is the geometry of BF₃ (boron trifluoride)?
Q4.Why do lone pairs compress bond angles?
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Common mistakes
Confusing electron geometry with molecular geometry. — Correct: Electron geometry includes lone pairs; molecular geometry only describes atom positions.
Forgetting that lone pairs must be counted when determining electron domains. — Correct: Always count bonding pairs AND lone pairs to find electron domains.
Assuming double and triple bonds count as multiple domains. — Correct: A double or triple bond counts as one electron domain.
Not realizing that geometry changes with lone pairs (e.g., thinking NH₃ is tetrahedral). — Correct: NH₃ is pyramidal because one domain is a lone pair, not bonding.
FAQ
What is the difference between electron geometry and molecular geometry?
Electron geometry includes lone pairs and bonding pairs; molecular geometry considers only bonding pair positions.
How do you count electron domains?
Count all bonding pairs (single, double, triple = one domain each) plus lone pairs on the central atom.
Why is PCl₅ trigonal bipyramidal and not trigonal planar?
5 electron domains (5 P-Cl bonds) arrange into two triangular pyramids stacked at their base.
Can a molecule be polar if it has polar bonds?
Not necessarily. If the polar bond dipoles cancel due to symmetry (linear CO₂), the molecule is nonpolar.