Bond polarity and dipole moment
A bond is polar when electrons are shared unequally, giving one end a partial negative charge (δ−) and the other a partial positive charge (δ+). A molecule is polar if its bond dipoles do not cancel.
Last updated
Analogy
Two people pulling a rope in different directions. If the pulls cancel, the box doesn't move. If not, it slides.
Example
CO₂ is linear, dipoles opposite: cancel.H₂O is bent, dipoles add up.So CO₂ is nonpolar, H₂O is polar.
In practice
Water is polar and dissolves salt. Carbon dioxide has polar bonds but is nonpolar overall because its dipoles cancel.
Practice
Flashcards
- Why is CO₂ nonpolar although C=O bonds are polar?It is linear, so the two bond dipoles point opposite ways and cancel.
Interview questions
- Use VSEPR to predict the shape and polarity of SO₂ and CO₂.
- SO₂ has three groups: two bonds and one lone pair, bent
- CO₂ has two groups, linear
- SO₂ dipoles do not cancel, so polar
- CO₂ dipoles cancel, so nonpolar
Questions people ask
Can a molecule with polar bonds be nonpolar?
Yes. If the molecule is symmetric, the bond dipoles point in opposite directions and cancel. CO₂, CCl₄ and BF₃ all have polar bonds but are nonpolar overall. Shape matters as much as the bonds themselves, so always check the geometry before calling a molecule polar.
How is dipole moment measured?
Dipole moment is charge times distance, measured in debye (D). It can be found by seeing how a substance behaves in an electric field. Water has a dipole moment of about 1.85 D, while symmetric molecules such as CO₂ have a measured value of zero.