Molecular solids, in their most common form, are excellent electrical insulators, meaning they generally do not conduct electricity. The simple answer to whether they conduct current is almost always no, although modern materials science has engineered some exceptions. This behavior stems from the fundamental difference in structure between molecular solids and conductive materials like metals. Understanding why requires a look at what molecular solids are and the basic requirements for electrical current flow.
Defining Molecular Solids
A molecular solid is a type of crystal lattice composed of discrete molecules, rather than individual atoms or ions. Common examples include ice (solid water), dry ice, and sugar. The atoms within each molecule are held together by strong covalent bonds, which involve the sharing of electrons.
The defining characteristic is the relatively weak intermolecular forces—such as Van der Waals forces and hydrogen bonds—that hold the individual molecules together. Because these forces are much weaker than the covalent, ionic, or metallic bonds found in other solids, molecular solids are typically soft and have low melting points.
The Requirements for Electrical Conduction
For any solid material to conduct an electric current, it must satisfy the fundamental requirement of having mobile charge carriers. These carriers must be able to move freely throughout the material’s structure when a voltage is applied; in most solids, these are electrons.
Metals are excellent conductors because their valence electrons are not tied to any single atom. Instead, they are “delocalized” and form a “sea” of electrons that can flow freely, providing a clear path for current. Electrical conduction requires electrons to have energy levels close enough to allow movement, often described in terms of an accessible conduction band.
Why Molecular Solids Resist Current
The structure of molecular solids directly prevents the formation of mobile charge carriers. The valence electrons are tightly held within the strong covalent bonds of each individual, neutral molecule, meaning the electrons are “localized” and confined to their specific molecular unit.
Since the intermolecular forces are weak, there is no easy path for electrons to jump from one molecule to the next. The energy required to free an electron and allow it to move through the intermolecular space is extremely high. This high energy requirement is scientifically described as a large “band gap,” which is characteristic of electrical insulators. This structural arrangement is why molecular solids, such as solid methane or sugar, function effectively as electrical insulators.
Rare Exceptions and Specialized Materials
While the general rule holds true, specialized fields of materials science have developed molecular solids that exhibit limited or functional conductivity. These exceptions are often found in organic semiconductors, which are molecular crystals or polymers built from carbon-based molecules. In these materials, complex molecular structures are designed to allow for some degree of electron movement or charge transfer. For example, certain organic dyes or polymers can have electrons that are partially delocalized across multiple atoms within a molecule, a characteristic known as pi-conjugation. This delocalization facilitates charge transfer when an external charge is injected, allowing the material to function as a semiconductor with low conductivity.
The conductivity of these organic solids can be significantly increased through a process called doping, where impurities are intentionally added to introduce mobile charge carriers. Under extreme conditions, such as high pressure, even normally insulating molecular solids can sometimes be forced into a conductive state. This specialized manipulation forces the molecules closer together, potentially changing the electronic structure enough to allow electron movement. However, for the vast majority of common molecular solids encountered in everyday life, their insulating nature remains a fundamental property of their molecular structure.