bond polarity

Not every covalent bond shares its electrons fairly. When two bonded atoms differ in their pull on electrons, the shared pair drifts toward the greedier atom, giving that end of the bond a slight negative charge and leaving the other end slightly positive. This lopsided sharing is bond polarity. Think again of two people sharing a blanket: if one tugs harder, the blanket — and the warmth — ends up on their side, even though neither has taken it outright.

Bond polarity is decided by the electronegativity difference between the two atoms. If the difference is essentially zero (as in C-C or C-H), the bond is nonpolar and the electrons are shared evenly. If the difference is moderate (C-O, C-N, C-Cl, O-H), the bond is polar covalent: chemists mark the more electronegative end with a small delta-minus and the other with a delta-plus, where delta means 'partial'. If the difference is very large (as between a metal and a nonmetal), the electron is essentially transferred and the bond is ionic. So polarity is a sliding scale from nonpolar through polar to ionic, set entirely by who pulls harder.

Bond polarity matters because it marks the spots where reactions happen. The slightly positive carbon of a polar C-O or C-halogen bond is exactly where electron-rich nucleophiles attack; the slightly negative oxygen is where electron-poor electrophiles and acidic protons are drawn. Polar bonds also build up into molecular dipole moments, which in turn govern boiling points and solubility. A subtle point: a molecule can contain polar bonds yet be nonpolar overall if those bond polarities point in opposite directions and cancel, as they do in symmetric CO2.