Chemical bonding, whether ionic or covalent, establishes the structure of materials and dictates their physical properties, including the ability to carry an electrical current. The way atoms interact—sharing or transferring electrons—determines if a material will act as a conductor or an insulator. The difference between ionic and covalent compounds regarding conductivity depends on the mobility of charged particles within their structure.
The Fundamental Requirement for Electrical Conductivity
Electrical conductivity requires a specific condition: the material must contain charged particles that are free to move throughout the substance. If these charged particles are absent or physically restricted from moving, the material will not conduct a current.
These mobile charged particles are either free-moving electrons or mobile ions. Metals are excellent conductors because they possess a “sea” of delocalized electrons that flow freely when voltage is applied. In materials without delocalized electrons, the current must be carried by charged atoms or molecules known as ions. This flow of mobile ions is the mechanism by which many solutions and molten compounds carry an electrical charge.
How Ionic Compounds Achieve Conductivity
Ionic compounds are formed by the complete transfer of electrons, typically between a metal and a nonmetal. This creates positively charged cations and negatively charged anions. These oppositely charged ions are held together by strong electrostatic forces, forming a rigid, repeating crystal lattice structure.
While ionic compounds contain charged particles, they are poor conductors in the solid state. This is because the ions are fixed within the lattice and cannot move to carry a current.
This changes when the compound is melted or dissolved in water. For example, when common table salt, Sodium Chloride (\(\text{NaCl}\)), is melted (molten) or dissolved in water (aqueous solution), the strong electrostatic forces holding the lattice are overcome. This breakdown frees the \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions, allowing them to move randomly through the liquid.
When an electric potential is applied, these mobile ions migrate toward the oppositely charged electrodes, effectively carrying the electrical charge. The conductivity of ionic compounds is entirely dependent on their physical state, transitioning from an insulator in solid form to a strong conductor in liquid or dissolved form.
Why Covalent Compounds Are Typically Insulators
Covalent compounds are formed when atoms, usually nonmetals, share valence electrons. This sharing results in the formation of neutral molecules, meaning there are no pre-existing ions to carry a current. Furthermore, the shared electrons are localized and tightly bound within the covalent bonds, preventing them from moving freely.
Since covalent substances consist of neutral molecules and lack mobile charge carriers, they are generally electrical insulators in all states—solid, liquid, or gas. Common covalent compounds like sugar or oil remain non-conductive even when melted or dissolved because the molecules stay intact and do not produce mobile ions.
There are rare exceptions, such as graphite, a form of carbon. In graphite’s layered structure, one valence electron from each carbon atom is delocalized, creating mobile electrons that allow it to conduct electricity. Other exceptions involve molecules that react with water to produce ions, such as hydrogen chloride gas (\(\text{HCl}\)) forming hydrochloric acid, but this is due to ionization, not an inherent property of the original molecule.
Direct Comparison and Final Conclusion
The ability of a substance to conduct electricity is determined by the presence and mobility of charged particles. Ionic compounds contain the necessary ions but only become conductive when these ions are freed from the rigid solid lattice by melting or dissolving. In their molten or aqueous state, ionic compounds are highly conductive due to the flow of mobile ions.
Covalent compounds are typically insulators because they consist of neutral molecules with no free electrons or mobile ions available in any state. Therefore, ionic compounds are substantially more conductive than covalent compounds, provided the ionic material is in a state that allows its ions to move.