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How to tell if a molecule is polar from its shape

A molecule is polar when its shape prevents the individual bond dipoles from canceling, leaving a net dipole moment. Look at the geometry: if the arrangement is symmetric (e.g., linear CO₂) the dipoles cancel; if it is asymmetric (e.g., bent H₂O) the molecule is polar.

Chemistry · Molecular geometry

Bond dipoles point from the less electronegative atom toward the more electronegative one. The vector sum of all bond dipoles gives the molecular dipole; a non‑zero sum means the molecule is polar.

Key geometric criteria

Check these geometric features:

  • Presence of lone‑pair electrons
  • Asymmetry of the shape
  • Difference in electronegativity of attached atoms

Determine polarity step‑by‑step:

  1. 1Draw the Lewis structure and identify bond dipoles.
  2. 2Assign vector direction from less to more electronegative atom.
  3. 3Add vectors using the molecular geometry; if they cancel, molecule is non‑polar.
  4. 4If a resultant vector remains, the molecule is polar.

Common shapes and expected polarity:

GeometryTypical polarity
Linear (e.g., CO₂)Non‑polar
Trigonal planar (e.g., BF₃)Non‑polar
Bent (e.g., H₂O)Polar
Trigonal pyramidal (e.g., NH₃)Polar
Tetrahedral (e.g., CH₄)Non‑polar

Check yourself

Which of the following molecules is non‑polar despite having polar bonds?

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