Molecular polarity
A molecule can contain polar bonds yet be nonpolar overall. Bond dipoles must be combined while considering geometry.
The essential idea
A molecule can contain polar bonds yet be nonpolar overall. Bond dipoles must be combined while considering geometry.
Example
In linear CO₂, the two C=O dipoles cancel. In bent H₂O they do not cancel, so the molecule is polar.
Mini problem
Does a polar bond always imply a polar molecule?
Show the solution
No. Molecular symmetry can make bond dipoles cancel.
Common mistake
Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.
Guided exercise
Compare CO₂ and NH₃: which molecule is polar overall?
Show the detailed solution
CO₂ is linear and its bond dipoles cancel. NH₃ is trigonal pyramidal, so the dipoles do not cancel and the molecule is polar.
Avoid this
A list of polar bonds is not enough: always return to three-dimensional geometry.