What is Molecular Geometry?
Molecular geometry describes the 3D spatial arrangement of atoms bonded to a central atom. Using VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs repel each other, forcing atoms into specific shapes that determine chemical and physical properties.
Molecular geometry is determined by counting electron pairs (bonding and lone) around the central atom. Electron pairs repel and arrange themselves to minimize repulsion, creating distinct 3D shapes: tetrahedral, trigonal planar, linear, pyramidal, and bent.
- •4 bonding pairs, 0 lone pairs
- •109.5° bond angles
- •3D: atom at center, 4 around
- •Example: methane, carbon tetrachloride
- •3 bonding pairs, 0 lone pairs
- •120° bond angles
- •Flat: atoms in a plane
- •Example: boron trifluoride, ethene
Step-by-step worked examples
Predict the geometry of NH₃ (ammonia).
N has 5 valence electrons, each H has 1. Lewis: N with 3 H atoms bonded + 1 lone pair. Total electron pairs = 4 (3 bonding + 1 lone). Electron geometry = tetrahedral (4 pairs). Molecular geometry = trigonal pyramidal (3 bonded atoms, 1 lone pair). Bond angle ≈ 107° (less than 109.5° due to lone pair repulsion).
What is the geometry of CO₂ (carbon dioxide)?
C has 4 valence electrons, each O has 6. Lewis: O=C=O (2 double bonds, 0 lone pairs on C). Total electron pairs = 2 (counting double bonds as one pair). Electron geometry = linear. Molecular geometry = linear. Bond angle = 180°.
Predict the shape of H₂O (water).
O has 6 valence electrons, each H has 1. Lewis: O with 2 H atoms bonded + 2 lone pairs. Total electron pairs = 4 (2 bonding + 2 lone). Electron geometry = tetrahedral (4 pairs). Molecular geometry = bent (2 bonded atoms, 2 lone pairs). Bond angle ≈ 104.5° (less than 109.5° due to two lone pairs).
Flashcards
Quick quiz
Q1.What geometry does a molecule with 4 electron pairs adopt?
Q2.PH₃ (phosphine) has 3 bonding pairs and 1 lone pair. Its molecular geometry is…
Q3.Which molecule is linear?
Q4.In water (H₂O), why is the bond angle 104.5° not 109.5°?
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Common mistakes
Confusing electron geometry with molecular geometry. — Correct: Electron geometry counts all pairs (bonding + lone); molecular geometry counts only bonded atoms. Both determine 3D structure.
Thinking lone pairs don't affect molecular shape. — Correct: Lone pairs strongly affect bond angles and 3D shape by repelling bonding pairs more than other bonding pairs repel each other.
Assuming all molecules with the same central atom have the same geometry. — Correct: Different numbers of bonding and lone pairs produce different geometries (e.g., CH₄ tetrahedral, but NH₃ pyramidal).
Applying VSEPR only to simple molecules. — Correct: VSEPR applies to any molecule with a clear central atom, even complex polyatomic ions.
FAQ
How do I predict molecular geometry?
Draw Lewis structure → count electron pairs (bonding + lone) → apply VSEPR → electron geometry → subtract lone pairs → molecular geometry.
Do double and triple bonds count as one pair in VSEPR?
Yes. A double bond = 1 electron pair for VSEPR purposes (both bonding electrons count as one pair for repulsion).
Why do lone pairs repel more than bonding pairs?
Lone pairs occupy more space; they're held by only one nucleus, so they exert greater repulsion on other pairs.
Can VSEPR predict polarity?
Partially—geometry affects polarity, but you also need electronegativity differences. A symmetric shape + identical atoms = nonpolar. Asymmetric + different atoms = polar.