Worked example 1
Predict the molecular shape and approximate bond angle of NH₃, and state whether it is polar.
Try it first: Count domains on nitrogen before naming anything.
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BondingVSEPRContent level: Core 20 min
VSEPR Shapes and Bond Angles
What you'll be able to do: Convert a domain count and lone pair count into a molecular shape with predicted bond angles.
Best after: Counting Electron Domains
Introduction
Once you can count domains, the shape follows from a short table, and lone pairs explain every deviation from the ideal angle.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Predict molecular shape from bonding domains and lone pairs
- Give ideal bond angles for each electron geometry
- Explain angle compression using lone pair repulsion
- Place lone pairs correctly in trigonal bipyramidal geometry
- Use shape and polarity together to predict a molecular dipole
Lesson
The shape table
With four domains: no lone pairs gives tetrahedral, one lone pair gives trigonal pyramidal, two lone pairs gives bent. With three domains: no lone pairs gives trigonal planar, one lone pair gives bent. With five domains: zero through three lone pairs give trigonal bipyramidal, seesaw, T-shaped and linear. With six domains: zero through two lone pairs give octahedral, square pyramidal and square planar.
Learn the domain geometry first, then subtract lone pairs to name the atom arrangement.
Why angles shrink
A lone pair is held by only one nucleus, so its electron cloud spreads wider than a bonding pair and pushes harder. Methane has ideal 109.5 degree angles, ammonia drops to about 107 degrees with one lone pair, and water drops to about 104.5 degrees with two. The same reasoning explains why PH₃ has angles near 93 degrees, closer to pure p orbital character than NH₃.
Repulsion strength ranks lone pair to lone pair, then lone pair to bonding pair, then bonding pair to bonding pair.
Shape and polarity
A molecule is polar when polar bonds fail to cancel. CO₂ is linear, so its two bond dipoles cancel and the molecule is nonpolar. Water is bent, so its dipoles add and the molecule is strongly polar. Symmetric shapes such as tetrahedral CCl₄ and square planar XeF₄ cancel; asymmetric shapes such as trigonal pyramidal NH₃ and seesaw SF₄ do not.
Key ideas
Rule
Lone pairs in a trigonal bipyramid
Lone pairs go equatorial because that position has only two neighbours at 90 degrees instead of three.
Rule
Ideal angles
Linear 180, trigonal planar 120, tetrahedral 109.5, trigonal bipyramidal 120 and 90, octahedral 90.
Definition
Bent shape
Two bonded atoms with one or two lone pairs on the central atom.
Key concept
SF₄
Five domains with one lone pair gives a seesaw shape and a net dipole.
Equation
Molecular shape rule
shape = f(bonding domains, lone pairs)
Worked examples
Worked example 2
Why is XeF₄ nonpolar even though every Xe-F bond is polar?
Try it first: Work out the domain count for xenon.
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Common mistakes
Calling water tetrahedral because it has four domains.
Why it's wrong: Molecular shape names the atoms only, and water has just two bonded atoms.
Check instead: Name the shape from bonded atoms, and use the domain count only to set the framework.
Putting a lone pair in an axial position of a trigonal bipyramid.
Why it's wrong: An axial lone pair would face three neighbours at 90 degrees instead of two.
Check instead: Always place lone pairs equatorially in five-domain systems.
Assuming polar bonds always give a polar molecule.
Why it's wrong: Symmetric geometry can cancel the individual bond dipoles.
Check instead: Add the bond dipoles as vectors using the predicted shape.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Molecular geometry follows from the number of domains and how many of them are lone pairs. Lone pairs occupy more angular space than bonding pairs, so they compress the remaining bond angles below the ideal value, and in a trigonal bipyramid they always take equatorial positions.
- Molecular shape comes from bonding domains after lone pairs are set aside
- Ideal angles are 180, 120, 109.5 and 90 depending on the domain count
- Lone pairs repel more strongly and compress bond angles
- Lone pairs take equatorial sites in trigonal bipyramidal geometry
- Symmetric shapes cancel bond dipoles and give nonpolar molecules
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.