Polar molecule

A polar molecule is a molecule with an uneven distribution of electric charge, so that one part is slightly positive and another part slightly negative. Malay: molekul berkutub · Chinese: 极性分子.

A molecule becomes polar when it contains atoms of different electronegativity that are arranged so their pulls do not cancel out. The more electronegative atom draws the shared electrons towards itself, giving it a small negative charge and leaving the other atom slightly positive. If the molecule is shaped so these small charges do not balance, the whole molecule has a positive end and a negative end and is described as polar. Polar molecules attract one another and are attracted to ions, which is why a polar solvent can pull an ionic lattice apart.

Example. Water is the most important polar molecule you meet. Its oxygen atom is much more electronegative than its hydrogen atoms, and the bent shape of the molecule means the charges do not cancel, so the oxygen end is slightly negative and the hydrogen end slightly positive. This is why water dissolves ionic compounds so well: the negative oxygen surrounds the cations and the positive hydrogens surround the anions.

Confusion to avoid. A molecule can contain polar bonds and yet not be polar overall if its shape makes the charges cancel, as in carbon dioxide, where the two opposite pulls line up and balance. So polarity depends on both electronegativity and shape, not electronegativity alone, and a symmetrical molecule is often non-polar even when its individual bonds are polar.

The polar molecule links electronegativity to solubility in this chapter and explains why ionic compounds dissolve in water but not in non-polar solvents, a property comparison used in SPM Chemistry.