polarity

Meaning: A bond is polar when two atoms differ in electronegativity, so the 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 moment. Geometry decides this. CO₂ and CCl₄ have polar bonds but are nonpolar overall because the dipoles cancel by symmetry. H₂O and CHCl₃ are polar because they do not cancel. Polarity drives "like dissolves like," boiling points (dipole-dipole forces and hydrogen bonding raise them), and how compounds behave in extraction and chromatography.

Equation: μ = q × d, where μ is the dipole moment (C·m), q is the magnitude of the partial charge (C), and d is the distance between the charges (m).

On the MCAT: Passages ask you to rank compounds by boiling point, solubility, or elution order. In normal-phase chromatography on silica, polar compounds stick more strongly to the polar stationary phase, so on TLC they travel a shorter distance (lower Rf), and on a column they typically elute later. In extraction, polar or charged species favor the aqueous layer. Solvent polarity also matters for substitution: polar protic solvents favor SN1, and polar aprotic solvents favor SN2.

Common confusion: Polar bonds do not guarantee a polar molecule. Draw the 3-D shape and add the dipole vectors. If they cancel, the molecule is nonpolar, even though each bond is polar.