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Ask “what shape is an ammonia molecule?” and you might hear two answers: “tetrahedral” and “trigonal pyramidal”. Both are correct — but they answer two different questions. Electron geometry (also called electron-domain or electron-pair geometry) describes how all regions of electron density around the central atom are arranged, including lone pairs. Molecular geometry (the molecular shape) describes where the atoms are. Mixing these up is one of the most common mistakes in VSEPR questions. This comparison makes the difference clear.
The two definitions
| Electron geometry | Molecular geometry | |
|---|---|---|
| What it describes | Arrangement of all electron domains (bonding pairs + lone pairs) around the central atom | Arrangement of the atoms (nuclei) around the central atom |
| Includes lone pairs? | Yes | No (lone pairs affect it but aren’t named) |
| Depends on | Total number of electron domains | Number of bonding domains and lone pairs |
| Possible names | Linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral | Those five, plus bent, trigonal pyramidal, seesaw, T-shaped, square pyramidal, square planar… |
| When they match | Always, if the central atom has no lone pairs | |
| What experiments see | Not directly observable | Measured by X-ray diffraction, spectroscopy |
An electron domain is any region of electron density around the central atom: a single bond, a double bond, a triple bond, or a lone pair. Each counts as one domain (see VSEPR and molecular geometry).
Why two geometries?
VSEPR theory works in two steps:
- Electrons decide the arrangement. All domains, including lone pairs, repel each other and spread out. This gives the electron geometry.
- Atoms decide the name. Experiments locate nuclei, not lone pairs, so the molecule’s shape is named from the atom positions only.
A good analogy: imagine four people standing around a small table, spaced evenly. If two of them are invisible, the two you can see still stand where they were — but you’d describe the visible arrangement differently.
The complete table
| Domains | Electron geometry | Lone pairs | Molecular geometry | Example |
|---|---|---|---|---|
| 2 | Linear | 0 | Linear | CO₂, BeCl₂ |
| 3 | Trigonal planar | 0 | Trigonal planar | BF₃, NO₃⁻ |
| 3 | Trigonal planar | 1 | Bent | SO₂, O₃ |
| 4 | Tetrahedral | 0 | Tetrahedral | CH₄, NH₄⁺ |
| 4 | Tetrahedral | 1 | Trigonal pyramidal | NH₃, H₃O⁺ |
| 4 | Tetrahedral | 2 | Bent | H₂O, H₂S |
| 5 | Trigonal bipyramidal | 0 | Trigonal bipyramidal | PCl₅ |
| 5 | Trigonal bipyramidal | 1 | Seesaw | SF₄ |
| 5 | Trigonal bipyramidal | 2 | T-shaped | ClF₃ |
| 5 | Trigonal bipyramidal | 3 | Linear | XeF₂, I₃⁻ |
| 6 | Octahedral | 0 | Octahedral | SF₆ |
| 6 | Octahedral | 1 | Square pyramidal | BrF₅ |
| 6 | Octahedral | 2 | Square planar | XeF₄, ICl₄⁻ |
Notice that the electron geometry column only ever contains five names, while the molecular geometry column contains many more.
Worked examples
Methane, CH₄
- Domains: 4 bonding, 0 lone.
- Electron geometry: tetrahedral. Molecular geometry: tetrahedral.
- No lone pairs, so they’re the same.
Ammonia, NH₃
- Domains: 3 bonding + 1 lone = 4.
- Electron geometry: tetrahedral.
- Molecular geometry: trigonal pyramidal — the three H atoms and the N form a pyramid; the lone pair occupies the fourth corner of the tetrahedron.
- Bond angle: about 107°.
Water, H₂O
- Domains: 2 bonding + 2 lone = 4.
- Electron geometry: tetrahedral.
- Molecular geometry: bent (V-shaped), angle 104.5° (see the shape of a water molecule).
Sulfur dioxide, SO₂
- Domains: 2 bonding (to O) + 1 lone = 3.
- Electron geometry: trigonal planar.
- Molecular geometry: bent, about 119°.
- Note: SO₂ and H₂O are both “bent”, but from different electron geometries.
Xenon difluoride, XeF₂
- Domains: 2 bonding + 3 lone = 5.
- Electron geometry: trigonal bipyramidal.
- Molecular geometry: linear — the three lone pairs sit around the equator, and the two F atoms are axial.
- Note: XeF₂ and CO₂ are both “linear”, from different electron geometries.
Xenon tetrafluoride, XeF₄
- Domains: 4 bonding + 2 lone = 6.
- Electron geometry: octahedral.
- Molecular geometry: square planar; the lone pairs are above and below the plane.
How the two geometries relate to other properties
| Property | Linked to |
|---|---|
| Hybridisation of the central atom | Electron geometry (4 domains → sp³, 3 → sp², 2 → sp) |
| Bond angles | Mainly electron geometry, adjusted by lone pairs |
| Molecular polarity | Molecular geometry (plus the lone pairs) |
| What X-ray diffraction measures | Molecular geometry |
So water is sp³ hybridised (because its electron geometry is tetrahedral), even though its shape is bent. Ammonia is sp³ too, despite being pyramidal (see how to determine hybridisation).
A worked comparison: three “tetrahedral” molecules
It helps to see how one electron geometry produces different shapes. Take three molecules that all have four electron domains around the central atom.
In tetrachloromethane, CCl₄, all four domains are C–Cl bonds. The electron geometry is tetrahedral and so is the shape. Because the four bond dipoles are identical and arranged symmetrically, they cancel, and the molecule is non-polar.
In phosphorus trichloride, PCl₃, three domains are P–Cl bonds and one is a lone pair. The electron geometry is still tetrahedral, but the atoms form a trigonal pyramid. The lone pair breaks the symmetry, so PCl₃ is polar.
In oxygen difluoride, OF₂, two domains are O–F bonds and two are lone pairs. The electron geometry is tetrahedral again, but the molecule is bent, with an angle of about 103°. It is also polar.
Three molecules, one electron geometry, three different shapes and two different polarities. The only thing that changed was how many of the four domains were lone pairs. That’s exactly why exam questions ask for the shape and why the lone pairs must be counted first. If you only remember one habit from this article, make it this: draw the Lewis structure, count every lone pair on the central atom, and only then name the shape.
Common exam mistakes
- Calling water “tetrahedral” when asked for the shape. The shape is bent; tetrahedral is its electron geometry.
- Calling ammonia “trigonal planar”. It’s trigonal pyramidal — the lone pair lifts the N out of the plane of the H atoms.
- Forgetting lone pairs when counting domains, which gives the wrong electron geometry entirely.
- Counting a double bond as two domains (CO₂ is linear, not tetrahedral).
- Assuming two molecules with the same shape have the same electron geometry (H₂O and SO₂ are both bent; CO₂ and XeF₂ are both linear).
Answering exam questions
If a question asks for “the shape”, “the molecular shape” or “the geometry of the molecule”, give the molecular geometry. If it asks for “the arrangement of electron pairs” or “electron-domain geometry”, give the electron geometry. When in doubt, give both, clearly labelled:
“Four electron pairs around the oxygen are arranged tetrahedrally (electron geometry). With two of them lone pairs, the molecule’s shape is bent, with a bond angle of about 104.5°.”
Key takeaways
- Electron geometry includes all electron domains; molecular geometry describes only atom positions.
- They’re the same when the central atom has no lone pairs.
- There are five electron geometries but many molecular shapes.
- Hybridisation follows electron geometry; polarity and measured structures follow molecular geometry.
- In exams, “shape” means molecular geometry.
For more practice, try VSEPR practice questions and how lone pairs change molecular shape.
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