Thermal conductivity quantifies how quickly heat flows through a material. When one end of a copper bar is placed in a flame, the other end gets hot almost immediately — copper's thermal conductivity is 401 W/(m·K), one of the highest of any metal. Silver edges it out at 429 W/(m·K), making it the best metallic heat conductor, while stainless steel languishes at about 15 W/(m·K), which is why stainless steel pan handles stay cooler. The mechanism in metals is straightforward: free electrons in the metallic bond carry kinetic energy from hot regions to cold ones at tremendous speed, and since metals have vast numbers of free electrons, they conduct heat exceptionally well.
But the single best thermal conductor is not a metal — it is diamond, at roughly 2,200 W/(m·K), over five times better than silver. Diamond has no free electrons; instead, its carbon atoms are locked in a rigid tetrahedral lattice with extremely strong covalent bonds. Heat travels through diamond as phonons — quantized vibrations of the crystal lattice — and diamond's stiff, lightweight structure lets phonons propagate with almost no scattering. This makes diamond invaluable as a heat sink for high-power laser diodes and semiconductor devices. Graphene, a single layer of carbon atoms, takes this even further — its in-plane thermal conductivity can exceed 5,000 W/(m·K), the highest of any known material.
At the opposite extreme, insulators like wood (~0.15 W/(m·K)), fiberglass (~0.04), and aerogel (~0.015) trap heat by minimizing conduction pathways. Air itself is a decent insulator at 0.025 W/(m·K), which is why many insulation materials work by trapping air in tiny pockets. Your winter jacket doesn't generate heat — it simply traps your body heat by preventing conduction and convection. Understanding thermal conductivity is critical for everything from designing CPU coolers and building insulation to choosing cookware materials and engineering spacecraft heat shields.