Polarity in chemistry refers to the unequal attraction of electrons in elements of a compound, resulting in a molecule with a negatively charged end and a positively charged end. The polarity of a molecule depends upon the electronegativity of the constituent elements of that molecule.
Elements that differ greatly in electronegativities will exert unequal attractions on electrons, creating a net difference in charge over the molecule. The polarity of individual bonds also determines a molecule’s shape as the unequal pull of electrons affects the spatial orientation of the molecules.
“Polarity, or action and reaction, we meet in every part of nature; in darkness and light; in heat and cold; in the ebb and flow of water; in male and female; in the equation of quantity and quality; in the fluids of the animal body.” — Ralph Waldo Emerson
Polarity is an important concept as it determines the number of physical properties of a substance. The polarity of a substance determines its surface tension, solubility, and melting/boiling point. Polar molecules interact in characteristic ways via hydrogen bonding and dipole-dipole interactions.
Polar Bonds And Polar Molecules
Not all elements attract electrons with the same strength. Elements with a high electronegativity pull harder on elements with low electronegativity. When two elements are bonded, the more electronegative element will exert a greater pull on the electrons. This causes the electrons to orient themselves closer to the more electronegative element. The movement of electrons creates a net difference in electrical charge and results in a molecule with a positively charged end and a negatively charged end.
Strictly speaking, kinds of chemical bonds can fall within two extremes, completely polar or completely non-polar. A bond that is completely polar involves one element taking an electron from another and is more correctly characterized as an ionic bond. Thus the term polar is most often reserved for covalently bonded compounds. When two elements have identical electronegativities, the electrons are pulled upon equally and the bond is non-polar. According to the Pauling scale, elements with an electronegativity difference of less than 0.5 make non-polar bonds and elements with an electronegativity difference between 0.5 and 2.0 make polar bonds. Any higher difference is considered an ionic bond.
“What is there about polarity that is matter becoming more complex?” — Norman Mailer
Most chemical compounds are made of more than two atoms and so consist of more than one chemical bond. Molecules can be polar either due to the polar bonds the have or due to an asymmetrical geometric arrangement of non-polar bonds. Conversely, a molecule can be overall non-polar even if it has polar bonds, provided those polar bonds are spatially oriented to cancel each other out. Polar molecules interact primarily through intermolecular dipole-dipole interactions. The differently charged ends of a polar molecule will attract the charged ends of other polar molecules. Unlike covalent or ionic bonds, dipole-dipole interactions are not true chemical bonds as they do not involve the sharing of electrons. How strong these attractive forces are determine how difficult it is to melt or boil something. The stronger the dipole-dipole interactions, the more kinetic energy required to break those attractions, so the more heat required to melt or boil that substance.
The polarity of a molecule also determines how well it will dissolve, and how readily it will be dissolved in another polar substance. Strongly polar substances are able to “steal” molecules via attractive forces, causing polar solids to dissolve in polar liquids.
Examples Of Polar Molecules
The most obvious example of a polar molecule is water. Water is composed of two hydrogen atoms and one oxygen atom. Oxygen is more electronegative than hydrogen and so exerts a stronger pull on the shared electrons. The conglomeration of electrons closer to the oxygen atom causes the oxygen end of the molecule to take on a negative charge while the hydrogen ends take on positive charges. The polarity of water molecules is responsible for a number of water’s physical properties.
Positively charged hydrogen ends are attracted to the negatively charged oxygen ends of other water molecules. This strong electrostatic intermolecular attraction explains the relatively high boiling point of liquid water as the strong dipole bonds require a lot of kinetic energy to overcome. Intramolecular attractions from hydrogen atoms is a special kind of dipole-dipole interaction called hydrogen bonding.
Interactions between polar water molecules also explain the phenomenon of surface tension on a body of water. At the surface of the water, polar interactions between the water molecules draw them together more strongly than polar molecules in the air above. The result is a “film” of highly attracted water molecules at the surface of the liquid. Water, in particular, has a high surface tension, strong enough to support insects and small animals.
Ammonia (NH3) is another common polar molecule. Ammonia has a pseudo-tetrahedral shape, with three base hydrogen atoms, a central nitrogen atom, and a single pair of electrons occupying the would-be 4th tetrahedral node. The presence of two unbonded electrons causes the molecule to be highly polar, as there is a distinct concentration of electric charges on the nitrogen end of the molecule. Because ammonia is highly polar, it will readily dissolve in a polar solvent such as water.
“The Swedish Academy of Sciences has seen fit, by awarding the Nobel Prize, to honour the method of producing ammonia from nitrogen and hydrogen.” — Fritz Haber
Ethanol, sometimes simply just called alcohol, is a polar solvent with a chemical formula of C2H5OH. Carbon-hydrogen bonds are non-polar so a molecule of ethanol is almost non-polar. The hydroxyl group attached to one of the carbon atoms is what gives ethanol its polarity. Both C-O and O-H bonds are polar in the direction of the oxygen atom. In addition, the oxygen atom has a lone pair of electrons opposite from the C-O and O-H bonds. Due to this polarity, ethanol is a versatile solvent used in a number of laboratory and industrial applications. The hydroxyl group at the end of ethanol also allows it to engage in hydrogen bonding like water.