Electrical conductivity is the ability of a material to allow an electric charge to flow through it, creating an electric current. This property varies across different substances, dictating their use in technology. Materials are broadly categorized as conductors, which allow charge flow easily, or insulators, which resist it strongly. Understanding what makes a compound conductive requires looking closely at the particles responsible for carrying the charge.
The Fundamental Difference in Charge Carriers
The primary distinction between conducting compounds lies in the type of particle that carries the electric charge, known as the charge carrier. In all cases, the movement of a charged particle is what constitutes electrical current. There are two main mechanisms for this charge transport: electronic conduction and ionic conduction.
Electronic conduction occurs when the flow of negatively charged electrons is responsible for carrying the current. This process is typical in metals and certain solid-state materials, where electrons are highly mobile. The atoms themselves largely remain fixed in their positions as the electrons move between them.
Ionic conduction, on the other hand, involves the movement of entire charged atoms or molecules, called ions. This mechanism requires the material to be in a liquid state, such as a solution or a molten compound, so the ions are physically free to move. Positive ions (cations) move toward the negative terminal, while negative ions (anions) move toward the positive terminal. This fundamental difference in the moving particle—subatomic electrons versus whole atoms/molecules—separates the two major classes of conductors.
Conductivity Driven by Electron Mobility
The most familiar conductors are those that rely on electronic conduction, primarily metallic elements and their alloys. Metals like copper, silver, and gold are excellent conductors because of their unique atomic structure. These materials are often modeled by the “sea of electrons” concept.
In this model, the valence electrons from each metal atom are not bound to a single nucleus but are instead delocalized, forming a mobile cloud that moves freely throughout the crystal lattice of fixed, positively charged metal ions. When an electric field is applied, these free-moving electrons are easily directed to flow, resulting in high electrical conductivity. This high mobility of electrons gives metals their characteristically low resistance.
Some non-metallic solids also exhibit high electronic conductivity, providing an exception to the metal-only rule. Graphite, a form of carbon, is one such example where a layer structure allows for electron mobility. Within each layer, electrons are delocalized, enabling them to move easily and allowing the substance to conduct electricity almost as well as some metals. This shows that the presence of delocalized electrons, regardless of the compound type, is the defining factor for this mode of conduction.
Conductivity Driven by Mobile Ions
The second major category of conductors includes compounds that rely exclusively on ionic conduction, often called electrolytes. These compounds are typically ionic salts, acids, or bases that must be in a state where their constituent ions are free to move. This state is achieved when the ionic compound is dissolved in a solvent, such as water, or when it is melted.
When an ionic compound like sodium chloride (table salt) is solid, the positive sodium ions and negative chloride ions are locked rigidly into a crystal lattice by strong electrostatic forces. Because the ions cannot physically move from their positions, the solid compound is a non-conductor. However, when the salt is dissolved, the solvent molecules break down this fixed lattice structure, releasing the individual ions.
Once the salt is molten or dissolved, the now mobile ions act as charge carriers, moving through the liquid to carry the electrical current. Molten salts achieve their conductivity when high temperature provides enough energy to overcome the strong ionic bonds, allowing the ions to flow. In both the solution and molten states, the movement of these charged atoms defines the material as an ionic conductor.
Defining Non-Conducting Materials (Insulators)
Materials that strongly resist the flow of electric charge are known as insulators, and their lack of conductivity stems from the absence of mobile charge carriers. Most common insulators, such as rubber, wood, glass, and many plastics, are composed of covalent compounds. In these compounds, the valence electrons are tightly held in localized bonds between specific atoms.
These electrons are not free to move throughout the structure, meaning there are no delocalized electrons to support electronic conduction. Furthermore, since they are not ionic compounds, they do not produce mobile ions when dissolved or melted. The electrons require a significant amount of external energy to break free from their fixed positions, which prevents them from carrying a current under normal operating conditions.
Falling between the high conductivity of metals and the high resistance of insulators are semiconductors, such as silicon. Semiconductors are materials that require specific conditions, like the addition of impurities through a process called doping or an increase in temperature, to gain enough mobile charge carriers to conduct electricity.