A molecule is polar when its electrons are shared unevenly so that one end is slightly negative and the other end is slightly positive, giving it an overall dipole.
Key requirements
For a molecule to be polar, both of these must be true:
- It contains at least one polar bond, meaning the bonded atoms have different electronegativities so electrons are pulled more toward one atom, creating partial charges.
- Its three‑dimensional shape does not let those bond dipoles cancel out, so there is a net dipole moment across the whole molecule.
Role of electronegativity
A bond becomes polar when the difference in electronegativity between two atoms causes unequal sharing of electrons, giving one end a partial negative charge and the other a partial positive charge. When such polar bonds are present and not symmetrically arranged, the molecule as a whole has separated charge and is therefore polar.
Role of molecular shape
Even if a molecule has polar bonds, it can be nonpolar if its geometry is symmetrical, because the individual bond dipoles cancel out. For example, carbon dioxide has polar C–O bonds but a linear shape, so the opposing bond dipoles balance and the molecule is nonpolar, whereas bent water and trigonal pyramidal ammonia have unbalanced dipoles and are polar.
