VSEPR theory
VSEPR stands for valence shell electron pair repulsion. Electron groups around a central atom (bonds and lone pairs) repel and spread as far apart as possible. The count of groups sets the geometry.
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Analogy
Balloons tied together at a knot push each other into the most spread-out arrangement.
Example
2 groups: linear (180°)3 groups: trigonal planar (120°)4 groups: tetrahedral (109.5°)5: trigonal bipyramidal. 6: octahedral
In practice
VSEPR predicts the bent shape of water (104.5°) and the pyramid shape of ammonia.
Practice
Flashcards
- What is the shape of a molecule with 3 bonds and 1 lone pair?Trigonal pyramidal, like NH₃.
Interview questions
- Use VSEPR to predict the shape and polarity of SO₂ and CO₂.
- SO₂ has three groups: two bonds and one lone pair, bent
- CO₂ has two groups, linear
- SO₂ dipoles do not cancel, so polar
- CO₂ dipoles cancel, so nonpolar
Questions people ask
Why is the H–O–H angle in water 104.5° and not 109.5°?
Water has four electron groups, which would give 109.5°. Two of them are lone pairs. Lone pairs take more space than bonding pairs, so they squeeze the O–H bonds closer together. That reduces the angle to about 104.5°, which is why water is bent and not tetrahedral.
What is the difference between electron geometry and molecular geometry?
Electron geometry counts every electron group, including lone pairs. Molecular geometry names only the arrangement of atoms. Ammonia has tetrahedral electron geometry, but because one group is a lone pair, its molecular geometry is trigonal pyramidal. Water works the same way: tetrahedral electron geometry, bent molecular geometry.