How does a molecule's polarity relate to its geometry?

A molecule's polarity is directly related to its geometry as the shape determines the distribution of electrical charges.

In more detail, the polarity of a molecule is determined by the difference in electronegativity between the atoms and the symmetry of the molecule. Electronegativity is a measure of how strongly an atom can attract a bonding pair of electrons. When two atoms in a molecule have different electronegativities, the bonding electrons will be more attracted to the atom with the higher electronegativity, creating a dipole moment, or a separation of charges. This results in one end of the molecule having a slight negative charge and the other end having a slight positive charge, making the molecule polar.

However, the overall polarity of a molecule also depends on its geometry. If a molecule is symmetrical, the dipole moments can cancel each other out, resulting in a nonpolar molecule. For example, carbon dioxide (CO2) is a linear molecule with two polar C=O bonds. However, because the molecule is linear, the two dipole moments are equal in magnitude but opposite in direction, cancelling each other out and making CO2 a nonpolar molecule.

On the other hand, if a molecule is not symmetrical, the dipole moments do not cancel out, resulting in a polar molecule. For instance, water (H2O) is a bent molecule with two polar O-H bonds. Because of the bent shape, the two dipole moments do not cancel each other out, making H2O a polar molecule.

In summary, the polarity of a molecule is determined by both the difference in electronegativity between the atoms and the symmetry of the molecule. A molecule can have polar bonds but still be nonpolar if the molecule is symmetrical and the dipole moments cancel out. Conversely, a molecule can be polar if it has polar bonds and the molecule is not symmetrical, resulting in a net dipole moment.