Polar molecules

Not all atoms pull on shared electrons equally. An atom’s greed for electrons is its electronegativity. When a bond links two atoms of different electronegativity, the electrons sit closer to the greedier one, giving it a slight negative charge and its partner a slight positive charge — a polar bond.

Whether the whole molecule is polar depends on its shape. Water is bent, so its two polar O–H bonds don’t cancel — water is strongly polar, which is why it dissolves so much. Carbon dioxide has polar bonds too, but its linear shape makes them cancel, so CO₂ is non-polar overall.