Worked example 1
How many electron domains surround the central atom in SO₂, and what is the electron geometry?
Try it first: Draw the Lewis structure and count total valence electrons first.
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BondingVSEPRContent level: Core 20 min
Counting Electron Domains
What you'll be able to do: Count the electron domains around a central atom and name the electron geometry they produce.
Introduction
Learn how to count electron domains around a central atom and turn that count into a VSEPR prediction. This lesson walks through linear, trigonal planar, tetrahedral, seesaw, and T-shaped geometries with worked examples, quick checks, and a step-by-step method you can use on any Lewis structure.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Define an electron domain and identify domains in a Lewis structure
- Explain why a double bond counts as a single domain
- Convert a domain count into an electron geometry
- Distinguish electron geometry from molecular geometry
- Count domains for ions with charges
- Predict seesaw and T-shaped molecular geometries from five electron domains
Lesson
What counts as a domain
An electron domain is a region of concentrated electron density around the central atom. Each lone pair is one domain, and each bond to another atom is one domain no matter whether it is single, double or triple. Carbon dioxide, O=C=O, has two domains on carbon even though four bonding pairs are involved.
Domains repel each other, so they arrange themselves as far apart as possible.
From count to electron geometry
Two domains spread to 180 degrees, which is linear. Three domains spread to 120 degrees in a plane, which is trigonal planar. Four domains take up a tetrahedron with angles of 109.5 degrees. Five domains give a trigonal bipyramid with axial and equatorial positions, and six domains give an octahedron with 90 degree angles.
Electron geometry versus molecular geometry
Electron geometry describes where all the domains sit. Molecular geometry describes only where the atoms sit, so lone pairs change the name even though they still occupy space. Water has four domains, so tetrahedral electron geometry, but only two bonded atoms, so a bent molecular shape.
Always name the geometry you were asked for; the two answers differ whenever a lone pair is present.
Bond angle exceptions: why lone pairs squeeze angles
The ideal angles, 109.5 degrees for four domains, 120 for three, and 180 for two, assume every domain is a bonding pair. Lone pairs are held closer to the central nucleus, so they spread out more and push bonding pairs together. Compare methane, ammonia, and water: all have four domains and tetrahedral electron geometry, but the H-C-H angle in CH₄ is the ideal 109.5 degrees, the lone pair in NH₃ squeezes H-N-H to about 107 degrees, and the two lone pairs in H₂O squeeze H-O-H to about 104.5 degrees. Multiple bonds repel a little more than single bonds too: in formaldehyde, H₂C=O, the H-C-H angle is slightly less than 120 degrees while each H-C=O angle is slightly more.
Exam shortcut: more lone pairs on the central atom means a smaller bond angle than the ideal.
Complex VSEPR shapes: seesaw and T-shaped
Five electron domains arrange as a trigonal bipyramid. When one of those domains is a lone pair, the atoms form a seesaw shape. The lone pair sits in an equatorial position because it repels the other domains most strongly there. Sulfur tetrafluoride, SF₄, is a classic example: four S–F bonds plus one lone pair give five domains and a seesaw molecular shape. When two of the five domains are lone pairs, both occupy equatorial positions and the three remaining atoms form a T-shaped molecule. Chlorine trifluoride, ClF₃, has three Cl–F bonds and two lone pairs, so its molecular shape is T-shaped.
In a trigonal bipyramid, lone pairs always go to equatorial positions first because they have fewer neighbors at 90°.
Key ideas
Definition
Electron domain
A lone pair or a bond (single, double or triple) counted as one region of electron density.
Rule
Multiple bonds
A double or triple bond occupies a single domain because all of its electrons lie between the same two atoms.
Rule
Domain to geometry
2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.
Key concept
Sulfite ion
SO₃ 2- has three bonded oxygens plus one lone pair on sulfur, so four domains and tetrahedral electron geometry.
Rule
Lone pairs and bond angles
A lone pair occupies more space than a bonding pair, so it pushes bonding pairs closer together and reduces bond angles. For example, CH₄ is 109.5°, NH₃ is 107°, and H₂O is 104.5°.
Key concept
Seesaw shape
SF₄ has five electron domains (four bonds + one lone pair). The lone pair occupies an equatorial position, giving a seesaw molecular shape.
Key concept
T-shaped molecule
ClF₃ has five electron domains (three bonds + two lone pairs). Both lone pairs occupy equatorial positions, leaving the three fluorine atoms in a T-shaped arrangement.
Equation
Domain count
domains = bonded atoms + lone pairs on the central atom
Worked examples
Worked example 2
What is the molecular shape of SF₄, and why does the lone pair occupy an equatorial position?
Try it first: Count the valence electrons, draw the Lewis structure, and then count domains around sulfur.
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Worked example 3
Predict the molecular shape of ClF₃.
Try it first: Count domains around chlorine before deciding on the shape.
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Common mistakes
Counting a double bond as two domains.
Why it's wrong: Both pairs sit in the same region between the same two nuclei, so they repel as one unit.
Check instead: Count bonded atoms, not bonds, and then add lone pairs.
Forgetting lone pairs on the central atom.
Why it's wrong: Lone pairs take up space and change the geometry even though they are invisible in a ball-and-stick model.
Check instead: Complete the Lewis structure before counting.
Reporting the electron geometry when the molecular shape was requested.
Why it's wrong: Lone pairs are counted in one and ignored in the other.
Check instead: Ask whether the question is about atom positions or domain positions.
Quoting the ideal angle for a molecule that has lone pairs.
Why it's wrong: Ideal angles like 109.5 degrees only hold when every domain is a bonding pair; lone pairs compress the bonding pairs and shrink the angle.
Check instead: Count the lone pairs first, then lower the ideal angle by roughly 2 to 2.5 degrees per lone pair.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
An electron domain is any group of electrons that occupies space around the central atom: a lone pair, a single bond, a double bond or a triple bond. Multiple bonds count as one domain. Two domains give linear electron geometry, three give trigonal planar, four tetrahedral, five trigonal bipyramidal and six octahedral. With five domains, one lone pair gives a seesaw shape and two lone pairs give a T-shaped molecule.
- A domain is a lone pair or a bond of any order
- Multiple bonds count as one domain
- Domain counts 2 through 6 map to linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral
- Electron geometry includes lone pairs; molecular geometry does not
- Lone pairs compress bond angles because they occupy more space than bonding pairs
- Always finish the Lewis structure before counting domains
- Five domains with one lone pair give a seesaw shape; five domains with two lone pairs give a T-shaped molecule
- In a trigonal bipyramid, lone pairs prefer equatorial positions
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.