Explicit science instruction
A complete polarity explanation should move through four linked representations: valence-electron accounting, Lewis structure, electron domains, and three-dimensional geometry.
Only after shape is established should bond-dipole directions be combined. This prevents the common error of labeling a molecule polar simply because it contains polar bonds.
Corrections belong in the evidence record. If the geometry changes after identifying a lone pair, revise the polarity claim rather than hiding the first model.
Vocabulary: representation chain; geometry; dipole; symmetry; revision.
Worked example with reasoning
For NH₃: Lewis structure gives three N–H bonds and one lone pair; four domains imply tetrahedral electron-domain geometry; atomic positions form a trigonal pyramid; the N–H dipoles do not cancel, so the molecule is polar.
Guided question
Write the full representation chain for H₂O from valence electrons to molecular polarity.
Independent work
- Submit representation chains for CO₂, BF₃, NH₃, and H₂O.
- Explain one case where polar bonds cancel.
- Explain one case where shape prevents cancellation.
- Record one corrected misconception from the week.
Spiral retrieval bridge
- From Week 07: how does a Lewis structure account for valence electrons?
- From Week 06: what does charge neutrality require in an ionic formula?
- From Week 05: how does electronegativity trend across a period?
- From Week 04: what evidence supports discrete electronic energy differences?
Misconception check
The correct polarity claim must follow from geometry, not from electronegativity difference alone.
Formative understanding check
What minimum reasoning chain should appear in a defensible molecular-polarity explanation?
Reveal response
Lewis/electron-domain evidence, molecular geometry, bond-dipole directions, and whether those dipoles cancel.
Public self-check only.
Mentor evidence conference
Ask Archaemenes to audit the representation chain and identify any unsupported leap.
Teacher guidance
Foundation: Complete one scaffolded representation chain.
Core: Defend polarity for four molecules.
Extended: Compare two molecules with the same electron-domain geometry but different molecular geometries and net dipoles.
Return to Friday Evidence