Explicit science instruction
VSEPR is a geometric model: electron domains around a central atom arrange to reduce repulsion. A single, double, or triple bond counts as one bonding domain; each lone pair counts as one lone-pair domain.
Two domains give a linear electron-domain arrangement, three give trigonal planar, and four give tetrahedral. Molecular geometry names the positions of atoms, so lone pairs can change the molecular shape while retaining the same electron-domain geometry.
The model predicts shape, not exact bond angles for every molecule. Lone pairs and multiple bonds can shift measured angles.
Vocabulary: electron domain; VSEPR; molecular geometry; lone pair; bond angle.
Worked example with reasoning
CO₂ has two bonding domains and no lone pairs on carbon, so the central atom is linear. H₂O has four electron domains around oxygen—two bonds and two lone pairs—so its molecular geometry is bent.
Guided question
Compare CH₄, NH₃, and H₂O: all have four electron domains around the central atom. Why do their molecular geometries differ?
Independent work
- Determine electron-domain count and molecular geometry for CO₂, BF₃, CH₄, NH₃, and H₂O.
- Draw one three-dimensional wedge/dash sketch.
- For one molecule, state where the VSEPR model may not give an exact measured angle.
Misconception check
A double bond counts as one electron domain in basic VSEPR; it does not create two separate directions around the central atom.
Formative understanding check
Why is NH₃ trigonal pyramidal rather than tetrahedral as a molecular shape?
Reveal response
Nitrogen has four electron domains but one is a lone pair, so only three atomic positions define the molecular geometry.
Public self-check only.
Mentor evidence conference
Ask Archaemenes to audit the representation chain and identify any unsupported leap.
Teacher guidance
Foundation: Use a domain-count table and shape reference.
Core: Predict five molecular geometries from Lewis structures.
Extended: Compare idealized and measured bond angles and explain why lone-pair repulsion can shift them.
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