Molecules with slightly positive and negative ends are known as polar molecules. These molecules have an uneven distribution of electron density, creating distinct partial positive (δ+) and partial negative (δ-) regions, even though the molecule as a whole remains electrically neutral.
How Molecules Become Polar
Molecular polarity arises from two main factors: the difference in electronegativity between bonded atoms and the overall molecular geometry. Electronegativity is an atom’s ability to attract shared electrons in a chemical bond. When two atoms with different electronegativities form a covalent bond, electrons are pulled more strongly towards the more electronegative atom, leading to unequal sharing and partial charges. For instance, in a water molecule (H₂O), oxygen is more electronegative than hydrogen, causing electrons in the O-H bonds to spend more time near the oxygen atom. This creates a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms.
The shape of a molecule, or its molecular geometry, then determines if these individual bond polarities result in an overall polar molecule. Even if a molecule contains polar bonds, a symmetrical geometry can cause partial charges to cancel, making the molecule nonpolar. Carbon dioxide (CO₂) is an example; its linear shape causes bond dipoles to cancel. In contrast, water has a bent shape, and ammonia (NH₃) a trigonal pyramidal shape, preventing bond dipoles from canceling, resulting in polar molecules.
How Polar Molecules Interact
The presence of distinct positive and negative regions allows polar molecules to interact with each other and with other substances through specific forces. Dipole-dipole interactions occur when the partial positive end of one polar molecule is attracted to the partial negative end of another polar molecule.
A particularly strong type of dipole-dipole interaction is hydrogen bonding. This occurs when a hydrogen atom, already bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, is attracted to a lone pair of electrons on another electronegative atom in a different molecule. Hydrogen bonds are stronger than typical dipole-dipole interactions and play a significant role in determining the properties of compounds like water.
The principle of “like dissolves like” governs the solubility of substances, directly related to molecular polarity. Polar substances tend to dissolve in other polar substances because their similar charge distributions allow for favorable intermolecular attractions. For example, sugar, a polar molecule, readily dissolves in water, which is also polar. Conversely, nonpolar substances, like oil, do not dissolve well in polar solvents such as water, but instead dissolve in other nonpolar solvents. This is why oil and water separate into distinct layers.
Polarity in Our World
Water’s polarity is foundational to its unique properties and its role in life. The bent shape of water molecules and their ability to form extensive hydrogen bonds contribute to its relatively high boiling point, allowing it to remain a liquid over a wide range of temperatures on Earth. Water’s strong solvent capabilities enable it to dissolve many polar and ionic substances, crucial for biological processes.
Molecular polarity also underpins numerous processes within biological systems. For instance, cell membranes are composed of phospholipids, which have a polar (hydrophilic) head and nonpolar (hydrophobic) tails. This dual nature causes phospholipids to arrange into a bilayer, forming the barrier that defines cells.
Similarly, the folding of proteins into their specific three-dimensional structures is heavily influenced by the arrangement of polar and nonpolar amino acids, with polar residues often found on the surface interacting with water and nonpolar residues tucked inside. DNA also exhibits polarity, important for its function.
Beyond biological systems, polarity is at work in many everyday substances, including alcohols, which are polar and mix well with water. Detergents and soaps exemplify the practical application of polarity. These cleaning agents are amphiphilic molecules, possessing both a polar (hydrophilic) head and a long nonpolar (hydrophobic) tail. The nonpolar tails attract and encapsulate nonpolar grease and dirt, while the polar heads interact with water, allowing the grime to be lifted and washed away.