Polarity

What is Polarity?

Some elements want to gain electrons more than others. In the module on bonding, distinctions between covalent and ionic bonds are discussed. Essentially, those differences are a function of element type (metal vs. nonmetal), atomic radius, and valence electrons. Significant differences between these properties for two different elements will likely result in an ionic bond. An example of such a bond is that formed between sodium (metal, 1 valence electron, relatively small) and chlorine (nonmetal, 7 valence electrons, also relatively small). In such a bond, chlorine wants an electron and sodium wants to give away an electron, forming an ionic bond. Once chlorine gains sodium's valence electron, it becomes the chloride ion, and there is a higher concentration of negative charge around this particle (18 electrons now surround a nucleus with 17 protons). The sodium ion that is also formed in this process is left with 10 electrons, but they still surround a nucleus with 11 protons. The loss of an electron more easily exposes the positively charged nucleus to surrounding particles, thus making this ion positive. When there is an identifiable positive and negative portion to a bond or molecule, that bond or molecule is said to be polar. Thus, the ionically bonded sodium chloride is made of a polar bond.

Ionic Bonds are Polar

Electronegativity differences can be used to classify bond types. Since no bond can have an electronegativity difference greater than 3.3 (Fr-F bond), a difference of 1.7 or greater is defined as ionic. Note that 1.7 is roughly 50% of 3.3, implying that a bond is ionic when the difference in electronegativity between two elements begins to show a notable difference. These bonds also exhibit a significant difference between the desire to give or take electrons, which classifies them as polar.

Polarity of Molecules

Polarity is the phenomenon where a molecule has a positive and negative pole as a consequence of the concentration of electrons around certain elements (creating a negative pole), and a lack of electron density (positive pole) around another element.  The diagram below shows the polarity of a water molecule.

Covalent compounds are broken down into two categories, nonpolar covalent and polar covalent.  In nonpolar covalent compounds, the difference in electronegativity is small enough that no discernible positive and negative region arises from an uneven distribution of electrons.  Polar covalent compounds, like water, have a distinct positive and negative pole.  Here, the negative region is concentrated around oxygen (the more electronegative element) and the positive region is located near the hydrogen atoms.