What is the Valence Shell Electron Pair Repulsion Law?
The Valence Shell Electron Pair Repulsion (VSEPR) law states that electron pairs in the valence shell of an atom repel each other and arrange to maximize their mutual separation. This principle explains why molecules adopt specific 3D geometries.
The VSEPR law: valence electron pairs repel each other; the geometry that maximises their separation is the actual molecular shape.
- •2 pairs: Linear
- •3 pairs: Trigonal planar
- •4 pairs: Tetrahedral
- •5 pairs: Trigonal bipyramidal
- •6 pairs: Octahedral
- •2 bonding: Linear
- •3 bonding: Trigonal planar
- •4 bonding: Tetrahedral
- •3 bonding + 1 lone: Pyramidal
- •2 bonding + 2 lone: Bent
Step-by-step worked examples
Apply VSEPR to CCl@sub{4} (carbon tetrachloride). Carbon is central with 4 chlorine atoms bonded.
Valence electrons on C: 4 All form C-Cl bonds Electron pairs: 4 bonding, 0 lone Geometry: Tetrahedral Bond angle: 109.5@deg Result: The 4 Cl atoms arrange symmetrically to minimize repulsion
Apply VSEPR to XeF@sub{2} (xenon difluoride). Xenon is central with 2 fluorine atoms bonded.
Valence electrons on Xe: 8 2 form Xe-F bonds; 6 remain as 3 lone pairs Electron pairs: 2 bonding, 3 lone Electron geometry: Trigonal bipyramidal Molecular geometry: Linear The 3 lone pairs occupy the equatorial positions; 2 F atoms are axial
Apply VSEPR to PCl@sub{3} (phosphorus trichloride).
Valence electrons on P: 5 3 form P-Cl bonds; 2 remain as 1 lone pair Electron pairs: 3 bonding, 1 lone Electron geometry: Tetrahedral Molecular geometry: Trigonal pyramidal The lone pair repels the 3 Cl atoms into a pyramid (107@deg Cl-P-Cl angle)
Flashcards
Quick quiz
Q1.In VSEPR, what is the primary force that determines molecular shape?
Q2.A central atom with 5 electron pairs adopts which electron geometry?
Q3.If a molecule has 3 bonding pairs and 2 lone pairs, its molecular geometry is…
Q4.Why do lone pairs occupy equatorial positions in a trigonal bipyramidal geometry?
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Common mistakes
Ignoring lone pairs when predicting geometry. — Correct: Lone pairs are electron pairs and must be counted and positioned.
Confusing electron geometry with molecular geometry. — Correct: Electron geometry includes lone pairs; molecular geometry is based on atoms only.
Thinking the strongest bonds make the shape. — Correct: Electron repulsion, not bond strength, determines shape.
Assuming all pairs repel equally. — Correct: Lone pairs repel more strongly than bonding pairs.
FAQ
What is the Valence Shell Electron Pair Repulsion law?
The law states that valence electron pairs repel each other; molecular shape is the geometry that maximises separation.
How does VSEPR predict molecular shape?
Count all valence electron pairs (bonding + lone), arrange them to maximise separation, then identify the molecular geometry.
Do lone pairs affect molecular shape?
Yes — they occupy space and cause repulsion, pushing bonded atoms into new arrangements.
What is the VSEPR law used for?
Predicting 3D molecular geometry, bond angles and polarity from a 2D Lewis structure.