Chemical bonding holds atoms together, forming compounds. All chemical bonds exist on a spectrum, but they are generally categorized into two primary types: ionic and covalent. These two bond types represent the extreme ends of how electrons are managed between atoms.
Defining Ionic and Covalent Bonds
An ionic bond involves the complete transfer of one or more valence electrons, typically between a metal and a nonmetal (e.g., sodium chloride, NaCl). The atom losing the electron becomes a positively charged cation, and the atom gaining the electron becomes a negatively charged anion. The resulting bond is a powerful electrostatic attraction between these oppositely charged ions.
In contrast, a covalent bond forms when electrons are shared between two atoms, usually two nonmetals. This sharing allows both atoms to achieve a stable electron configuration. Covalent bonds are either nonpolar (shared equally) or polar (shared unequally).
Determining Bond Type with Electronegativity
Electronegativity is the quantitative metric used to classify bonds. It measures an atom’s ability to attract a shared pair of electrons toward itself within a chemical bond. The Pauling scale is the most commonly used system to assign numerical values to this property.
Bond type is predicted by calculating the difference in electronegativity (Delta EN) between the two bonded atoms. A small or zero Delta EN suggests the electrons are shared equally, resulting in a nonpolar covalent bond. When the difference is very large, typically greater than 1.7, the bond is considered predominantly ionic, indicating electron transfer.
What Polarity Means for Chemical Bonds
The term “polar” describes an unequal distribution of electron density, primarily applied to covalent bonds where sharing is uneven. This unequal sharing occurs when there is a moderate difference in electronegativity (Delta EN) between the atoms, pulling the shared electrons closer to the more electronegative atom. The result is a separation of charge within the bond, creating a partial negative charge (delta-) and a partial positive charge (delta+).
This charge separation creates an electric dipole moment, which measures the bond’s polarity. Nonpolar covalent bonds, such as those in chlorine gas, have zero Delta EN, meaning the electrons are shared equally and no permanent dipole exists.
Classifying Ionic Bonds: The Definitive Answer
The question of whether an ionic bond is polar or nonpolar is technically a misapplication of the terms because the concept of polarity is defined by the unequal sharing of electrons in a covalent bond. An ionic bond does not involve sharing; it involves the complete, rather than partial, transfer of electrons. This transfer results in a full unit of charge separation between the cation and the anion.
The electrostatic attraction between the fully charged ions represents the absolute maximum possible separation of charge. If a spectrum existed for polarity, the ionic bond would sit at the extreme end, representing 100% ionic character and zero covalent character. Most chemists avoid labeling an ionic bond as simply “polar” and instead describe it as having a significant “ionic character” or as being the most extreme case of charge separation.