Molecular Shapes from Electron Pair Repulsion
Electron pairs around a central atom push each other as far apart as they can, and that alone predicts the shape. A lone pair pushes harder than a bonding pair, which is why water is bent and not straight.
What a learner can do afterwards
- Predict the shape and bond angle of a simple molecule from the number of bonding and lone pairs
- Explain why ammonia and water have smaller bond angles than methane
- Draw a three-dimensional representation using wedges and dashes rather than a flat sketch
1 · Read
Electron pairs around a central atom push each other as far apart as they can. Count the bonding plus lone pairs and you can predict the shape and its angles. Sketch the Lewis structure first, since dots and lines give you the pair count.
Methane has four bonding pairs and no lone pairs, giving a tetrahedron with angles near 109.5 degrees. Draw it in 3D: a solid wedge means a bond coming out at you, and a dashed line means one going back. Never settle for a flat sketch.
A lone pair pushes harder than a bonding pair. Ammonia carries one lone pair, so its angle shrinks below methane. Water carries two, so it bends tighter still. Single, double and triple bonds each count as one direction when you count. If your sketch and your pair count disagree, recheck the Lewis structure first.
Work every shape the same way. Draw the Lewis structure, count pairs around the central atom, then pick the shape. If your sketch and your pair count disagree, recheck the Lewis structure first.
Draw pairs first, count them next, and let lone pairs squeeze the shape you sketch in 3D.
Rewritten readings were written for this dataset.
More to read
Predict shape from electron regions
Electron regions repel one another, so they spread out as much as possible around the central atom. Count bond directions and lone pairs to estimate geometry and angles. Methane's four bonds, without lone pairs, form a tetrahedral arrangement with angles of about 109.5 degrees. Show its depth using solid wedges towards the viewer and dashed bonds away, rather than a flat arrangement.
Use a Lewis structure to count regions
Prepare a Lewis structure before predicting geometry. Its lines represent bonds; dots show unshared electrons. A lone pair generally pushes on nearby bonding regions more strongly than a bonding region does. Methane, ammonia and water each have four electron regions around the central atom, with zero, one and two lone pairs respectively. Their bond angles become progressively smaller.
Count bond directions for shape
Atoms form covalent bonds by sharing electrons, creating molecules. In shape predictions, a single bond occupies one electron region, as does a double or triple bond. Grouping multiple bonds this way makes counting manageable: use their directions rather than the number of shared electrons.
Connect orbital models with geometry
Hybridisation models combine atomic orbitals to describe directional regions for bonds and lone pairs. Wedge-and-dash drawings show these directions in three dimensions. The orbital model, angles and predicted geometry should agree. If your drawing conflicts with your electron-region count, check your Lewis structure before proceeding.
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2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.