polar

Meaning: A bond is polar when the two atoms differ enough in electronegativity that shared electrons sit closer to the more electronegative atom, creating partial charges δ+ and δ−. A molecule is polar when its bond dipoles add to a net dipole, which depends on shape. H₂O is bent and polar. CO₂ is linear, so its two C=O dipoles cancel and the molecule is nonpolar. C–H bonds are usually treated as nonpolar.

Equation: μ = qd, where μ is the dipole moment (C·m, often reported in debye), q is the magnitude of the partial charge (C), and d is the separation between the charges (m). For a molecule, μ is the vector sum of the bond dipoles.

On the MCAT: Expect "like dissolves like" questions. Polar solutes dissolve in polar solvents such as water, and nonpolar solutes dissolve in nonpolar solvents such as hexane. Polar molecules have stronger dipole-dipole attractions, so they usually have higher boiling points than nonpolar molecules of similar mass. In normal-phase chromatography on silica, polar compounds stick to the stationary phase and elute later. Biochemistry uses the same idea for polar amino acid side chains.

Common confusion: Polar bonds do not guarantee a polar molecule. Symmetric molecules such as CCl₄, BF₃ and CO₂ have polar bonds but no net dipole. Draw the 3D shape and check whether the dipoles cancel. Polar also does not mean ionic, since the charges are partial.