Explicit science instruction
A polar bond has unequal electron sharing and can be represented by a bond-dipole vector pointing toward the more electronegative end.
Overall molecular polarity depends on the vector sum of all bond dipoles. Geometry therefore matters: polar bonds can cancel in a symmetric molecule.
A molecule can be nonpolar overall without having nonpolar bonds. Conversely, an asymmetric arrangement of polar bonds can produce a net molecular dipole.
Vocabulary: bond dipole; molecular dipole; polarity; electronegativity; vector sum.
Worked example with reasoning
CO₂ contains two polar C=O bonds, but its linear geometry places equal bond dipoles in opposite directions, so they cancel. H₂O is bent, so its O–H dipoles do not cancel and the molecule is polar.
Guided question
Explain why BF₃ and NH₃ can contain polar bonds but differ in overall molecular polarity.
Independent work
- Draw bond-dipole arrows for CO₂, H₂O, BF₃, and NH₃.
- Predict whether each molecule is polar or nonpolar overall.
- Write one sentence linking symmetry to dipole cancellation.
Misconception check
Bond polarity and molecular polarity are not the same property; the latter depends on both bond dipoles and molecular geometry.
Formative understanding check
Can a molecule with polar bonds be nonpolar overall?
Reveal response
Yes. If geometry is sufficiently symmetric, the bond-dipole vectors can cancel.
Public self-check only.
Mentor evidence conference
Ask Archaemenes to audit the representation chain and identify any unsupported leap.
Teacher guidance
Foundation: Use arrow diagrams on supplied molecular shapes.
Core: Predict overall polarity from bond polarity plus shape.
Extended: Treat bond dipoles as vectors and explain qualitative cancellation using symmetry.
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