How does the shape of a molecule factor into its molecular polarity?

Answer 1

Let's begin by first understanding what polarity means. The polarity of a molecule arises from the electronegativity difference of the atoms that participate in covalent bonds that are found within the molecule. In order for a bond to be a "polar" covalent bond, there needs to be a significant difference in electronegativity between the atoms that participate in the bond. For instance, an example of a polar bond is a bond between an oxygen and a hydrogen atom. Oxygen is significantly more electronegative than hydrogen, and within the O-H bond, the oxygen atom has a partial negative charge, while the hydrogen atom has a partial positive charge. The charge difference stems from the pull that the oxygen atom has on the electrons within the shared O-H bond. Due to the oxygen atoms' electronegativity, it has a stronger pull on the electrons within the O-H bond, causing the oxygen atom to develop a partial negative charge and the hydrogen atom to develop a partial positive charge. This leads to the formation of a polar covalent bond when an oxygen and hydrogen atom bond to one another.

The shape of a molecule factors into it's molecular polarity because, depending on the shape of the molecule, the individual dipoles in the molecule can cancel out. I'll provide 2 examples to show my point.

Within a water molecule, there are 2 polar covalent bonds: 2 hydrogen atoms each bond to 1 oxygen atom. Each of these bonds have an individual dipole moment that results from the pull that the oxygen atom has on the electrons within the bonds. These individual dipole moments reinforce each other and lead overall to a net dipole moment that is in the direction of the oxygen atom because of the bent shape of the water molecule.

In our second example, a carbon dioxide molecule has a linear shape. The carbon atom is the central atom and is bonded to 2 oxygen atoms. Within the individual carbon-oxygen double bonds, the dipole is in the direction of each oxygen atom because oxygen is more electronegative than carbon. Therefore, the individual dipole moments are in opposite directions to one another and this leads to a cancellation of the net dipole moment of the molecule.