Why do methane, ammonia and water have different shapes?
You will be able to: Distinguish electron-domain and molecular geometry when lone pairs are present.
Why do methane, ammonia and water have different shapes?
CH₄, NH₃ and H₂O each have four domains around their central atom. Yet their atom arrangements are tetrahedral, pyramidal and bent. The difference is how many domains contain visible bonded atoms.
A useful starting point: How do electron domains predict a molecule’s shape? →
Words and symbols before equations
- Molecular geometry
- Arrangement named using atom positions, excluding lone pairs from the shape name.
- Lone-pair domain
- A region of electron density at the center without a bonded atom in that direction.
- Bond-angle compression
- A reduction from an ideal angle due to unequal domain repulsions.
What this picture assumes
CH₄ uses the ideal tetrahedral angle. NH₃ and H₂O use approximate example angles 107° and 104.5°. Lone-pair markers indicate domains only, not measured electron clouds; terminal lone pairs are omitted.
Read the picture in three steps
- Read the species and labels first. A Lewis line represents two electrons; a spatial stick indicates connectivity. Use the stated quantities and units for numerical comparisons.
- CH₄: 4 central domains; tetrahedral; bond angles 109.47° (about 109.5°).
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Four domains give a tetrahedral electron-domain starting model. Replacing a bonding domain with a lone pair does not remove that region of electron density.
NH₃ has three N–H bonds and one N lone pair: tetrahedral domains but trigonal-pyramidal molecular geometry. H₂O has two bonds and two lone pairs: tetrahedral domains but bent molecular geometry.
Lone pairs often exert greater repulsion near the central atom than bonding domains. In these examples the bond angles are about 107° for NH₃ and 104.5° for H₂O, below methane’s approximately 109.5°. These are approximate example values, not universal angles for every pyramidal or bent molecule.
| Molecule | Electron-domain geometry | Molecular geometry |
|---|---|---|
| CH₄ | Tetrahedral; 4 bonds | Tetrahedral |
| NH₃ | Tetrahedral; 3 bonds + 1 lone pair | Trigonal pyramidal |
| H₂O | Tetrahedral; 2 bonds + 2 lone pairs | Bent |
A worked example, step by step
Use eight valence electrons to predict NH₃ shape.
- Draw three N–H single bonds using six electrons.
- Put the remaining two electrons in one N lone pair.
- Count four domains: three bonding plus one lone pair.
- The atom-only shape is trigonal pyramidal, with H–N–H about 107° in this example.
Do not erase lone pairs before predicting shape. Exclude them only when naming the atom arrangement.
Does water have a linear shape because only two atoms surround oxygen?
Compare with an explanation
No. Oxygen’s two lone pairs also occupy domains, giving a bent molecular shape.
Predict. Change one thing. Explain.
Switch from CH₄ to NH₃ to H₂O. Hold the total domain count at four. Track the number of lone pairs, the atom-only shape and the example angle; rotate to inspect depth.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
CH₄: 4 central domains; tetrahedral; bond angles 109.47° (about 109.5°).
CH₄ uses the ideal tetrahedral angle. NH₃ and H₂O use approximate example angles 107° and 104.5°. Lone-pair markers indicate domains only, not measured electron clouds; terminal lone pairs are omitted.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using electron accounting, electrostatic interactions or spatial geometry. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionCompare CH₄ and H₂O. Give domain counts, lone-pair counts and molecular geometries, then explain why the two visible O–H directions do not give a linear water molecule.
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Compare with the answer and four-point rubric
- 1 point: Both have four central domains.
- 1 point: CH₄ has zero lone pairs; H₂O has two.
- 1 point: CH₄ is tetrahedral; H₂O is bent.
- 1 point: The two O lone pairs influence the spatial arrangement even though they are excluded from the molecular shape name.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What distinguishes the two geometry names?
Electron-domain geometry includes lone pairs; molecular geometry names atom positions.
RECALL 2Why is ammonia pyramidal?
Three bonded atoms occupy part of a four-domain arrangement with one lone pair.
RECALL 3Are all bent molecules 104.5°?
No. That approximate value is for water, not every bent species.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why do methane, ammonia and water have different shapes?
- CH₄: 4 bonds, 0 lone pairs → tetrahedral.
- NH₃: 3 bonds, 1 lone pair → trigonal pyramidal.
- H₂O: 2 bonds, 2 lone pairs → bent.
Remember: Do not erase lone pairs before predicting shape. Exclude them only when naming the atom arrangement.
Conditions: CH₄ uses the ideal tetrahedral angle. NH₃ and H₂O use approximate example angles 107° and 104.5°. Lone-pair markers indicate domains only, not measured electron clouds; terminal lone pairs are omitted.
Refresh Kid · AP Chemistry Unit 2 · Objectives 2.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.7, objectives 2.7.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 2: Compound Structure and Properties, Topics 2.1–2.7. The focused lesson breakdown is Refresh Kid’s editorial sequence. Models and original practice are teaching materials, not official AP questions. Numerical potential curves, ion comparisons and orbital-alignment indices state their approximations. Five- and six-domain shapes are included; d-orbital hybridization and molecular-orbital diagrams are not required here. GitHub’s 3D website examples, including the Three.js Mars camera-control example, informed the use of rotatable scenes. Our scientific geometry and viewer code are original; no repository artwork or tutorial code was copied. The self-hosted Three.js library retains its MIT license. Camera rotation does not alter chemistry. See also the official clarifications.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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