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
Answer: No. Molecular symmetry can make bond dipoles cancel.
Explanation: Bond dipoles must be combined while considering geometry. Before moving on, explain “Molecular polarity” in two sentences without rereading the page.
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
Check: A yes/no answer is complete only if you can justify the direction of the relation or state the mechanism that requires it.
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
Answer: CO₂ is linear and its bond dipoles cancel. NH₃ is trigonal pyramidal, so the dipoles do not cancel and the molecule is polar.
Explanation: In bent H₂O they do not cancel, so the molecule is polar. Linear CO₂ cancels its two C=O dipoles; bent H₂O does not.
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
Avoid this
A list of polar bonds is not enough: always return to three-dimensional geometry.