Why bond polarity and molecular geometry determine molecular polarity?

Question

Cameron Andrews · Accepted Answer

Because molecular polarity results from the VECTOR sum of the individual bond dipoles. Vectors have magnitude and direction, so polarity is in part a function of geometry.

Take carbon tetrafluoride, #CF_4#; the #C-F# bonds all show charge separation in that fluorine polarizes electron density towards itself (i.e. a #C-F# bond is polar!). Nevertheless, if we take the vector sum of these individual bond dipoles, the indvidual vectors sum to zero. The same reasoning applies to #"C"Cl_4#, carbon tetrachloride, and #CHCl_3#, chloroform. Carbon tetrachloride is a non-polar solvent in that the bond dipoles will sum to zero (they must, because of the molecular symmetry, tetrahedral with #109.5##""^@# bond angles. On the other hand, chloroform has a degree of molecular polarity and is thus a polar solvent, inasmuch as the individual bond dipoles DO NOT sum to zero, and there is a resultant molecular polarity. In summary, molecular polarity is determined by two factors: (i) bond polarity, which is measured by the difference in electronegativity between the bound atoms; and (ii) the VECTOR sum of the bond dipoles.

Cooper Anderson · Answer

undefined Bond polarity and molecular geometry determine molecular polarity because the distribution of electron density around a molecule is influenced by both factors. The presence of polar bonds, where there is a significant difference in electronegativity between atoms, results in an uneven distribution of electron density within the molecule. Additionally, the overall shape or geometry of the molecule affects how these polar bonds interact with each other, leading to either a net dipole moment (polar molecule) or no net dipole moment (nonpolar molecule). Therefore, understanding both bond polarity and molecular geometry is crucial in determining whether a molecule is polar or nonpolar.