Electron Geometry vs. Molecular Geometry: Examples and a Quiz
Water has tetrahedral electron geometry and bent molecular geometry. Both answers describe the same molecule. The difference is what you include: electron geometry describes the arrangement of bonding regions and lone pairs around the central atom; molecular geometry describes the arrangement of the atoms.
For water, both labels follow from one count: two O–H bonds and two lone pairs on oxygen. Keeping that count beside your answer helps prevent the two geometry names from getting mixed up.
Count regions around one atom
VSEPR stands for valence-shell electron-pair repulsion. In this model, regions of electron density around a central atom arrange themselves to reduce repulsion. An electron domain is one such region. “Electron-pair geometry,” “electron-domain geometry,” and “electron geometry” refer to the same classification here.
The method below covers ordinary closed-shell examples in introductory chemistry: their Lewis structures have paired electrons. It predicts local geometry around a chosen atom, rather than the overall shape of a large molecule.
Use this sequence:
- Count the valence electrons and draw a valid Lewis structure, including lone pairs. Add one electron per negative charge or subtract one per positive charge.
- Choose the atom whose local geometry you're describing.
- Count its bonding domains. Each attached atom contributes one domain, whether the connection is a single, double, or triple bond.
- Count the lone pairs on that chosen atom. Each pair contributes one more domain.
- Write AXE notation. Use the total domain count to name the electron geometry, then name the shape formed by the atoms in that arrangement.
In AXₘEₙ, A is the central atom, X counts attached atoms, and E counts lone pairs on A. Thus AX₂E₂ means two attached atoms and two lone pairs: four domains total. E₂ represents four nonbonding electrons, grouped into two pairs. When there are no lone pairs, you can omit E₀.
Lone pairs still influence molecular geometry. You leave them out when naming the shape formed by the atoms; you don't remove their effect on the bonds. The University of Calgary's explanation of electron-pair geometry and molecular shape illustrates this distinction.
A VSEPR chart for the first answer
The total domain count gives the electron geometry. It doesn't always give the molecular geometry.
| Domains around A | Electron geometry | Ideal angles between domain directions |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | About 109.5° |
| 5 | Trigonal bipyramidal | 90°, 120°, 180° |
| 6 | Octahedral | 90°, 180° |
These are ideal reference arrangements, often called parent geometries. Actual bond angles can differ because domains don't all repel equally. VSEPR is an approximate model, so a geometry name alone doesn't supply an exact experimental angle.
CH₄, NH₃, and H₂O share four domains
Start with methane, CH₄. Carbon contributes four valence electrons and the four hydrogens contribute one each: eight in total, all used in four C–H single bonds. Carbon has no lone pairs: four bonding domains plus zero lone pairs gives AX₄. The electron geometry and molecular geometry are both tetrahedral.
Ammonia, NH₃, also has eight valence electrons: five from nitrogen and three from hydrogen. Three N–H bonds use six; the remaining two form one lone pair on nitrogen. Three bonding domains plus one lone pair gives AX₃E. The electron geometry is tetrahedral, and the molecular geometry is trigonal pyramidal, with nitrogen at the apex and the three hydrogens forming the base.
Water, H₂O, has eight valence electrons too: six from oxygen and two from hydrogen. Two O–H bonds use four, leaving four electrons as two lone pairs on oxygen. Two bonding domains plus two lone pairs gives AX₂E₂. Its electron geometry is tetrahedral; its molecular geometry is bent.
| Molecule | Bonding domains + central lone pairs | AXE | Electron geometry | Molecular geometry |
|---|---|---|---|---|
| CH₄ | 4 + 0 | AX₄ | Tetrahedral | Tetrahedral |
| NH₃ | 3 + 1 | AX₃E | Tetrahedral | Trigonal pyramidal |
| H₂O | 2 + 2 | AX₂E₂ | Tetrahedral | Bent |
For these molecules, the bond angles decrease from about 109.5° in methane to 107° in ammonia and 104.5° in water. Within VSEPR, stronger repulsion from lone pairs explains the compression. Don't transfer water's 104.5° angle to every bent molecule. OpenStax's molecular structure lesson distinguishes ideal arrangements from these distortions.
CO₂: two double bonds still mean two domains
Carbon dioxide has 16 valence electrons: four from carbon and six from each oxygen. Its Lewis structure is O=C=O. The two double bonds use eight electrons; the remaining eight form two lone pairs on each oxygen, leaving none on carbon.
At carbon, each C=O connection is one bonding domain. The count is 2 + 0 → AX₂ → linear electron geometry → linear molecular geometry, with an O–C–O angle of 180°.
Counting four drawn bond lines would give the wrong domain count. Adding the oxygen lone pairs would describe the wrong atom. Circle the central carbon on paper if that helps keep the count local.
Five and six domains need the same reasoning
SF₄ has 6 + 4 × 7 = 34 valence electrons. Four S–F single bonds use eight, and completing the four fluorine octets uses another 24 as lone pairs. The remaining two electrons form one lone pair on sulfur. Around sulfur, 4 + 1 gives AX₄E: trigonal bipyramidal electron geometry and seesaw molecular geometry.
A trigonal bipyramid has three equatorial positions in a plane and two axial positions above and below it. The sulfur lone pair takes an equatorial position: there it has two neighboring domains at 90°, compared with three in an axial position. This placement reduces the lone pair's close interactions with bonding domains.
XeF₄ has 8 + 4 × 7 = 36 valence electrons. Four Xe–F single bonds use eight and the fluorine lone pairs use 24, leaving four electrons as two lone pairs on xenon. The count is 4 + 2 → AX₄E₂ → octahedral electron geometry → square planar molecular geometry. The xenon lone pairs occupy opposite positions; the four fluorines lie in a square with xenon at its center.
Four attached atoms can therefore give tetrahedral CH₄, seesaw SF₄, or square planar XeF₄. The missing information is the central atom's lone-pair count.
Try the quiz before reading the answers
For each prompt, write “bonding domains + central lone pairs,” AXE notation, and both geometry names. Then answer the question or correct the proposed answer.
- HCN has the Lewis structure H–C≡N, with one lone pair on nitrogen. Describe the geometry around carbon.
- NH₄⁺ has four N–H single bonds. Count its valence electrons, including the charge. Does it have the same molecular geometry as NH₃?
- A central atom has two attached atoms and one lone pair. Is its electron geometry bent?
- XeF₂ has two Xe–F bonds and three lone pairs on xenon. Is its electron geometry linear?
- A student gives every AX₂E₂ molecule a bond angle of exactly 104.5°. What should change in that answer?
Answers and the mistake each catches
- 2 + 0; AX₂; linear; linear. The triple bond is one domain. Nitrogen's lone pair doesn't enter carbon's count.
- 4 + 0; AX₄; tetrahedral; tetrahedral. NH₄⁺ has 5 + 4 − 1 = 8 valence electrons, all in its four bonds. NH₃ is trigonal pyramidal because nitrogen has a lone pair. Recount the electrons when the formula or charge changes.
- 2 + 1; AX₂E; trigonal planar; bent. “Bent” names the atom arrangement. Three total domains select the trigonal planar parent geometry.
- 2 + 3; AX₂E₃; trigonal bipyramidal; linear. The three lone pairs occupy equatorial positions, leaving two opposite axial bonds. A linear molecular shape doesn't prove there are only two domains.
- 2 + 2; AX₂E₂; tetrahedral; bent. About 104.5° belongs to water. Report the geometry separately from a molecule-specific angle; the ideal tetrahedral reference is about 109.5°.
Practice the step you missed
For another round, the VSEPR molecular geometry flashcards provide 78 text-only cards for introductory chemistry. On a geometry question, write your domain count and both names before revealing the answer. Keep drawing Lewis structures on paper alongside the cards, especially if electron counting was the step you missed.
You can also make a front/back card from one error: “Why isn't CO₂ tetrahedral?” on the front; “Each double bond is one domain; carbon has two domains and no lone pairs” on the back. The getting-started guide explains how to create cards and review due items. For broader course material, the advanced chemistry flashcard guide covers how to turn chemical reasoning into practice prompts.