In formaldehyde the dipoles for the C - H bonds are much smaller (weaker) than the C = O dipole (based on the difference in EN values). The dipole arrows in the C - H bonds are directed at carbon, the more electronegative atom. The dipole arrow for the C=O bond is directed at oxygen, the more electronegative atom. The polarity of the C = O bond cannot be “subtracted” out or "cancelled" by the polarity of the C - H bonds, because the dipoles do not cancel. As a result, an overall dipole exists along the C=O bond and the molecule formaldehyde is a polar molecule.
