A polar molecule can be understood as a tiny magnet, possessing a distinct positive and negative end. This separation of charge arises from an uneven distribution of electrons within the molecule, creating two electrically charged regions called poles. This characteristic dictates how these molecules interact with each other and with other substances, which is important for many chemical and biological processes.
The Foundation of Polarity
The origin of a molecule’s polarity is electronegativity, which is the measure of an atom’s ability to attract shared electrons in a chemical bond. When two atoms with different electronegativity values form a covalent bond, the electrons are not shared equally. The atom with the higher electronegativity pulls the shared electrons closer, acquiring a slight negative charge, while the other atom develops a slight positive charge. This creates a polar covalent bond, a prerequisite for a polar molecule.
However, the presence of polar bonds alone is not sufficient to make an entire molecule polar. The molecule’s three-dimensional shape is also a determining factor. For a molecule to be polar, it must have an asymmetrical geometry, such as a bent or pyramidal shape. This asymmetry ensures that the individual bond polarities do not cancel each other out, resulting in an overall net dipole moment for the molecule.
A counterexample is carbon dioxide (CO2). The bonds between carbon and oxygen are polar because oxygen is more electronegative than carbon. Yet, CO2 is a nonpolar molecule because it has a linear and symmetrical shape. The two polar bonds point in opposite directions, effectively canceling each other out.
Common Examples of Polar Molecules
Water (H2O) is the most well-known example of a polar molecule. The oxygen atom is significantly more electronegative than the hydrogen atoms, causing it to pull the shared electrons closer. This gives the oxygen atom a partial negative charge and the hydrogen atoms partial positive charges. The molecule’s bent shape prevents these charges from canceling out, making water highly polar.
Ammonia (NH3) is another polar molecule. Nitrogen is more electronegative than hydrogen, creating polar N-H bonds. The molecule has a trigonal pyramidal shape due to a lone pair of electrons on the nitrogen atom. This asymmetrical structure ensures that the individual bond dipoles do not cancel, resulting in a net dipole moment and making ammonia a polar molecule.
Ethanol (C2H5OH) demonstrates polarity due to the presence of its hydroxyl (-OH) group. The oxygen atom is much more electronegative than the hydrogen atom it is bonded to, creating a significant polar bond. This part of the molecule gives ethanol its polar character, allowing it to interact with other polar substances like water.
Properties and Interactions
The polarity of molecules dictates many of their physical properties and how they interact. A guiding principle is “like dissolves like,” which means polar substances dissolve in polar solvents, while nonpolar substances dissolve in nonpolar solvents. This is why salt and sugar, both composed of polar components, readily dissolve in water. The positive and negative ends of the water molecules attract the charged parts of the solute, pulling them apart.
The attraction between polar molecules gives rise to intermolecular forces, such as dipole-dipole interactions and hydrogen bonds. Hydrogen bonds, a stronger type of dipole-dipole force, occur when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. These strong attractions between water molecules are responsible for its high surface tension, the property that allows small insects to walk on its surface.
These intermolecular forces also explain why polar molecules have higher boiling points than nonpolar molecules of a similar mass. More energy is required to overcome the strong attractions between polar molecules to transition them from a liquid to a gas. The cohesion of water (molecules sticking to each other) and adhesion (molecules sticking to other polar surfaces) are also direct results of these forces.
The Contrast with Nonpolar Molecules
In contrast to polar molecules, nonpolar molecules have an even distribution of electrical charge. This can happen when bonds are between atoms with very similar electronegativity, or when the molecule has a symmetrical shape that causes bond dipoles to cancel out. In molecules like oxygen (O2) or nitrogen (N2), the electrons are shared equally because the atoms are identical.
Methane (CH4) is an example of a nonpolar molecule due to its symmetrical tetrahedral shape. Although the individual carbon-hydrogen bonds have a slight polarity, their symmetrical arrangement means their effects cancel each other out, resulting in no net dipole moment. Similarly, oils and fats are largely composed of nonpolar hydrocarbon chains.
The properties of polar and nonpolar molecules explain why oil and water do not mix. Water molecules are strongly attracted to each other through hydrogen bonds and exclude the nonpolar oil molecules. This is because the attraction between water molecules is much stronger than any potential attraction between water and oil, resulting in the two liquids separating into distinct layers.