What is VSEPR Theory?
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular shape by assuming electron pairs repel each other. The geometry that minimises repulsion determines whether molecules are linear, trigonal planar, tetrahedral or other shapes.
VSEPR theory states that electron pairs (bonding and lone) around a central atom repel each other, arranging to maximize separation and minimize repulsion, giving rise to the molecule's 3D shape.
- 1↓Count valence electronsIdentify all valence electrons on the central atom and bonded atoms
- 2↓Arrange electron pairsPlace electron pairs (bonding + lone) as far apart as possible
- 3↓Minimize repulsionThe arrangement that maximises separation minimises repulsion
- 4Determine geometryThe resulting 3D arrangement defines the molecular shape
Step-by-step worked examples
Predict the shape of CH@sub{4} (methane). Carbon has 4 valence electrons and forms 4 C-H bonds.
Central atom: C Electron pairs: 4 bonding pairs, 0 lone pairs Total electron pairs: 4 Geometry: Tetrahedral (109.5@deg bond angles) Shape: All 4 hydrogens point to the vertices of a tetrahedron
Predict the shape of NH@sub{3} (ammonia). Nitrogen has 5 valence electrons and forms 3 N-H bonds.
Central atom: N Electron pairs: 3 bonding pairs, 1 lone pair Total electron pairs: 4 Electron geometry: Tetrahedral; Molecular geometry: Trigonal pyramidal The lone pair occupies space, pushing the 3 hydrogens into a pyramid shape
Predict the shape of H@sub{2}O (water). Oxygen has 6 valence electrons and forms 2 O-H bonds.
Central atom: O Electron pairs: 2 bonding pairs, 2 lone pairs Total electron pairs: 4 Electron geometry: Tetrahedral; Molecular geometry: Bent The 2 lone pairs repel the 2 bonds, giving a 104.5@deg bond angle
Flashcards
Quick quiz
Q1.How many electron pairs surround the carbon in CH@sub{4}?
Q2.What is the molecular geometry of NH@sub{3}?
Q3.Why is the H-O-H bond angle in water ~104.5@deg, not 109.5@deg?
Q4.Which molecule has a linear geometry?
The full card deck, worked steps and AI-tutor support for “What is VSEPR Theory?” are in Notek — study by hand before your exam.
Common mistakes
Counting only bonding pairs. — Correct: Include lone pairs too — they occupy space and cause repulsion.
Thinking molecular geometry = electron geometry. — Correct: Lone pairs affect geometry but are not counted in the final molecular shape.
Assuming all 4-pair arrangements are tetrahedral. — Correct: 4 pairs can give tetrahedral (4 bonds), pyramidal (3 bonds + 1 lone), or bent (2 bonds + 2 lone).
Ignoring the effect of lone pairs on bond angles. — Correct: Lone pairs repel more, shrinking the bond angle.
FAQ
What is VSEPR theory?
VSEPR predicts molecular shape by assuming electron pairs repel and arrange to maximize separation.
How do I predict molecular geometry using VSEPR?
Count all electron pairs (bonding + lone) around the central atom, arrange them to minimize repulsion, then identify the shape.
What is the difference between electron geometry and molecular geometry?
Electron geometry includes lone pairs; molecular geometry counts only atoms (not lone pairs).
Does VSEPR work for all molecules?
It works well for main-group compounds; transition metal complexes are more complex.