Nonmetals generally do not conduct heat well and are typically classified as thermal insulators. These elements lack the typical properties of metals, such as luster, malleability, and the ability to easily transfer energy. While all matter transfers some heat, the mechanism nonmetals use is vastly different and highly inefficient compared to metallic elements.
The Mechanism of Heat Transfer in Metals
Heat transfer in metals is efficient due to their unique atomic structure. Metals possess a “sea” of highly mobile, delocalized electrons that are not bound to any single atom. When a metal object is heated, these free electrons absorb kinetic energy and quickly travel throughout the material. This rapid movement and collision of energized electrons serves as the primary means of transferring thermal energy.
Because the same mobile electrons carry both electrical current and thermal energy, metals that are excellent electrical conductors, like copper and silver, are also excellent thermal conductors. The electronic contribution to heat transfer in metals is dominant, making the thermal energy transferred by vibrating atoms negligible in comparison.
How Nonmetals Conduct Heat
Nonmetals lack the free electrons necessary for rapid heat transfer because their valence electrons are tightly bound in covalent or ionic bonds. Since the electronic mechanism is unavailable, nonmetals rely on lattice vibration. When heat is applied, atoms at the hot spot vibrate with greater intensity, passing this energy to their neighbors in a chain reaction.
These quantized units of vibrational energy are referred to as phonons. The movement of phonons is significantly less efficient than the movement of free electrons because the energy transfer relies on the physical “bumping” of neighboring atoms. This process is often scattered or disrupted within the structure, meaning thermal energy moves through a nonmetal hundreds of times slower than through a metal.
Practical Differences in Thermal Conductivity
The difference in heat transfer mechanisms results in a dramatic difference in material performance. Typical metals, such as aluminum or copper, have thermal conductivity ranging from 50 to 400 Watts per meter-Kelvin (W/(m·K)). In contrast, solid nonmetals like wood, glass, or plastic typically have values between 0.01 and 1 W/(m·K).
This low conductivity makes nonmetals valuable as thermal insulators in real-world applications. For example, materials like fiberglass trap air (a nonmetal gas with low conductivity) and are used in building walls to prevent heat exchange. Additionally, the low conductivity of plastic and wood allows them to be used as handles on metal cookware, protecting the user from heat.
Notable Nonmetal Exceptions
While nonmetals are generally poor conductors, a few exceptions demonstrate the importance of atomic structure. Diamond, an allotrope of carbon, is a nonmetal that exhibits room-temperature thermal conductivity exceeding 2,000 W/(m·K). This value is higher than that of the best-conducting metals, such as copper.
This exceptional performance occurs because diamond has an extremely rigid and perfectly ordered crystal lattice structure. This structure allows phonons to travel with minimal scattering or resistance, enabling the vibrational energy to move at high speed. Other carbon allotropes, such as graphene, also show high thermal conductivity for the same reason.