Molecular solids are classified as electrical insulators because they do not permit the flow of electric current. This poor conductivity stems directly from their unique physical structure, which lacks the mobile components required for electrical conduction. Understanding this requires examining the molecular arrangement of these materials and the fundamental requirements for carrying a charge.
Defining the Structure of Molecular Solids
Molecular solids are composed of individual, discrete molecules held together in a fixed structure. The fundamental units are neutral molecules, such as water in ice or sucrose in table sugar. These molecules are not chemically bonded to one another to form a single continuous network.
The forces binding these molecules are known as intermolecular forces. These forces—including London dispersion forces, dipole-dipole interactions, and hydrogen bonds—are significantly weaker than the covalent or ionic bonds found in other solids. Because these interactions are weak, molecular solids generally have low melting points and are soft.
How Electricity Travels Through Materials
Electrical conductivity requires mobile charge carriers that can move freely when an electrical potential is applied. A material can conduct electricity through two primary mechanisms.
The first mechanism involves the movement of delocalized electrons, as seen in metals like copper or silver. In these materials, valence electrons form a “sea of electrons” throughout the crystal lattice. This allows electrons to be easily promoted into the conduction band, enabling current flow.
The second mechanism relies on the physical movement of mobile ions, occurring in molten ionic compounds or electrolytes. Solid table salt is an insulator because its ions are locked in a rigid structure. Melting or dissolving the salt frees the charged ions, which then migrate toward the oppositely charged electrode, carrying the current.
Why Molecular Solids Are Electrical Insulators
Molecular solids fail to meet either fundamental requirement for electrical conduction, explaining their classification as insulators. The electrons are tightly held within the strong covalent bonds of each individual molecule. These electrons are localized and are not shared between neighboring molecules, preventing them from forming the “sea” necessary for metallic conduction.
The energy gap between the occupied valence band and the empty conduction band is very large in molecular solids. Overcoming this gap requires substantial energy, which is not provided by a standard electrical potential. Consequently, the electrons remain confined within their respective molecules and cannot traverse the material.
Furthermore, the constituent molecules of these solids are electrically neutral, meaning they do not carry a net charge. Since there are no mobile ions available, the second mechanism for conduction is also unavailable. For example, solid ice is a poor conductor, and when sugar is dissolved in water, it breaks apart into neutral molecules, ensuring the resulting solution remains non-conductive.
Exceptions: Molecular Materials That Conduct Electricity
While most molecular solids are insulators, a few specialized materials have been engineered to exhibit conductivity. These exceptions require modifying the typical molecular structure to allow for electron mobility. Such materials are classified as organic semiconductors or conductors.
Conductive Polymers
Conductive polymers, such as polyacetylene, achieve conductivity through conjugation, an extended system of alternating single and double bonds. This structure creates overlapping molecular orbitals that allow electrons to become delocalized along the polymer chain. The electron can then “hop” from one molecule to the next, especially when the material is treated with dopants to introduce charge carriers.
Charge-Transfer Salts
Another exception is charge-transfer salts, like tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). These compounds form when an electron partially transfers between two different molecules, resulting in a stack of charged molecules or radical ions. The close stacking and partial charge allow for the formation of conductive pathways where electrons can move, bridging the gap between insulators and metals.