Covalent compounds are formed when atoms, typically nonmetals, share electrons. This sharing process creates discrete, electrically neutral molecules, which are the building blocks of the compound. Electrical conductivity is determined by the mobility of charged particles within a substance’s structure. Covalent compounds are generally electrical insulators because their unique bonding structure does not provide the necessary mobile charge carriers.
The Requirements for Electrical Flow
For any material to carry an electrical current, it must contain charged particles that are free to move throughout its bulk. This movement of charge defines electrical conductivity. Mobile charge carriers take one of two forms, depending on the material’s composition and physical state.
One form involves ions, which are atoms or molecules with a net positive or negative charge. These charged particles must be physically mobile, such as when an ionic compound is melted or dissolved in a solvent. The other mechanism involves electrons that are not tightly bound to a single atom but are delocalized and able to drift freely across the material’s structure. Materials that allow this free movement of charges are classified as conductors.
Why Covalent Structures Prevent Charge Movement
The shared nature of electron pairs in a covalent bond is the primary reason these compounds are poor conductors. Electrons are tightly held in a specific region between the two bonded atoms. These shared electrons are localized and are not free to move away from their fixed positions to carry an electrical current.
This arrangement means that even when a voltage is applied, no delocalized electrons are available to move through the substance. When covalent compounds dissolve or melt, they typically do not break apart into charged ions; the entire, neutral molecule remains intact. Therefore, there are no mobile ions to transport the charge through the liquid. The absence of both free ions and delocalized electrons fundamentally prevents the flow of electricity through most covalent materials.
The Fundamental Difference from Conductive Materials
The insulating behavior of covalent compounds is clearer when contrasted with high-conductivity materials. Metallic substances are excellent conductors due to their “sea of electrons” bonding model. In metals, valence electrons are delocalized and shared among all positive metal ions, allowing them to flow freely when an electric field is applied.
Ionic compounds, formed by the transfer of electrons, use a different mechanism. Solid ionic compounds do not conduct electricity because their charged ions are locked into a rigid crystal lattice. However, when an ionic solid is melted or dissolved, the lattice breaks down, freeing the positive cations and negative anions to move. This mobility of charged ions enables the flow of current, making molten or aqueous ionic compounds good conductors.
Notable Exceptions to Non-Conductivity
While the general rule is that covalent compounds do not conduct electricity, a few exceptions exist. The most well-known exception is graphite, a form of carbon that is a giant covalent network solid. Graphite’s structure consists of carbon atoms arranged in flat layers, where each atom is bonded to only three others.
This bonding leaves one valence electron per atom delocalized above and below the layers. These delocalized electrons are free to move within the layers, allowing graphite to conduct electricity effectively, unlike other covalent network solids such as diamond.
Ionization in Solution
Another exception involves highly polar covalent compounds, such as hydrogen chloride (HCl). Although pure, gaseous HCl is a non-conductor, dissolving it in water causes ionization. The strong polarity of the bond causes the molecule to dissociate completely, forming mobile hydrogen ions (H+) and chloride ions (Cl-). In this case, the resulting solution of mobile ions, rather than the pure covalent compound itself, conducts the electrical current.