How do electron domains predict a molecule’s shape?
You will be able to: Count central-atom domains and distinguish a 2D projection from a 3D arrangement.
How do electron domains predict a molecule’s shape?
Four objects placed around a center have more room if they spread into three dimensions. The four C–H bonds of methane point toward tetrahedral directions, not the corners of a flat square.
A useful starting point: Why can one Lewis drawing be insufficient? →
Words and symbols before equations
- VSEPR
- Valence-shell electron-pair repulsion, a model for arranging electron-density regions.
- Electron domain
- One bond direction or one lone pair around a chosen atom.
- Electron-domain geometry
- Arrangement of all domains around the center.
- Bond angle
- Angle between two bonds at the central nucleus, in degrees.
What this picture assumes
Sphere-and-stick positions show geometry, not orbitals or atom sizes. Two-dimensional drawings are projections. Bond angles refer to the actual 3D coordinates. Terminal lone pairs are omitted from geometry models.
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.
- CO₂: 2 central domains; linear; bond angles 180°.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Start with the Lewis diagram. Count each single, double or triple bond as one domain, and each lone pair as one. Several electron pairs in a multiple bond occupy one direction.
Two domains favor opposite directions (180°). Three favor a planar triangle (120°). Four favor a tetrahedron (about 109.5°). These are ideal arrangements, not angles measured from a flattened drawing.
Methane has four bonding domains and no lone pairs at C, so its molecular and electron-domain geometries are both tetrahedral. Rotate the model to see why a projection can hide a bond or distort an apparent angle.
A worked example, step by step
Predict geometry around carbon in CO₂ and CH₄.
- CO₂ has two double bonds at carbon, giving two domains, not four.
- Two domains give a linear arrangement with O–C–O = 180°.
- CH₄ has four single bonds, giving four domains.
- Four domains with no lone pairs give tetrahedral CH₄, H–C–H ≈ 109.5°.
Count directions of electron density, not the number of bond lines or electrons.
If a tetrahedron looks almost flat from one viewpoint, did its bond angles change?
Compare with an explanation
No. The camera projection changed; the molecule’s geometry did not.
Predict. Change one thing. Explain.
Switch among CO₂, BF₃ and CH₄. Predict the domain count and shape first, then rotate the optional 3D model. Camera rotation changes the projection, not the labeled bond angle.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
CO₂: 2 central domains; linear; bond angles 180°.
Sphere-and-stick positions show geometry, not orbitals or atom sizes. Two-dimensional drawings are projections. Bond angles refer to the actual 3D coordinates. Terminal lone pairs are omitted from geometry models.
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 questionPredict the central geometries of CO₂, BF₃ and CH₄ from domain counts. Explain why a multiple bond does not count as two domains.
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Compare with the answer and four-point rubric
- 1 point: CO₂: two domains, linear, 180°.
- 1 point: BF₃: three domains, trigonal planar, 120°.
- 1 point: CH₄: four domains, tetrahedral, about 109.5°.
- 1 point: A multiple bond contains more pairs but one direction of electron density.
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 1Where should shape prediction begin?
With a valid Lewis diagram.
RECALL 2What is a domain?
A bond direction or a lone pair around the selected atom.
RECALL 3What does a rotatable model reveal?
Spatial arrangement hidden by a single projection.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do electron domains predict a molecule’s shape?
- 2 domains → linear, 180°.
- 3 → trigonal planar, 120°.
- 4 → tetrahedral, about 109.5°.
Remember: Count directions of electron density, not the number of bond lines or electrons.
Conditions: Sphere-and-stick positions show geometry, not orbitals or atom sizes. Two-dimensional drawings are projections. Bond angles refer to the actual 3D coordinates. Terminal lone pairs are omitted from geometry models.
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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