Metals conduct heat and electricity well because their atoms share their outermost electrons in a way that creates a vast, mobile pool of charge carriers. These “free” electrons are not bound to any single atom. Instead, they drift through the metal’s crystal lattice, carrying electrical charge when a voltage is applied and ferrying thermal energy from hot regions to cold ones. This shared-electron arrangement is unique to metallic bonding and is the single biggest reason metals behave so differently from wood, glass, or plastic.
What Free Electrons Actually Do
In a chunk of copper or aluminum, the metal atoms sit in a regular, repeating grid. Each atom gives up one or more of its outermost electrons, which then roam freely throughout the entire structure. The atoms left behind carry a positive charge, and together they form a rigid scaffold held in place partly by the attraction to that electron cloud surrounding them.
When you connect a battery across a piece of copper wire, the voltage pushes those free electrons in one direction. Because billions upon billions of them can move simultaneously, current flows easily. Materials like rubber or ceramic have almost no free electrons available, so pushing current through them takes enormous voltage, if it works at all.
Heat conduction works through the same cast of characters. When you hold one end of a metal spoon over a flame, the atoms at that end vibrate faster. Free electrons in the hot zone pick up kinetic energy from those vibrations, zip through the lattice, and collide with atoms in the cooler region, transferring energy as they go. Electrons are tiny and fast, so they carry heat far more efficiently than the lattice vibrations alone could. That is why a metal pan handle gets painfully hot in seconds while a wooden spoon handle stays comfortable.
Why the Same Metals That Conduct Electricity Well Also Conduct Heat Well
There is a remarkably clean pattern here: the metals that top the list for electrical conductivity, silver, copper, gold, and aluminum, also top the list for thermal conductivity. That is not a coincidence. In the mid-1800s, physicists Gustav Wiedemann and Rudolph Franz noticed that the ratio of a metal’s thermal conductivity to its electrical conductivity, at a given temperature, is roughly the same constant for all metals. The relationship became known as the Wiedemann-Franz law, and the constant itself is called the Lorenz number.
The reason the law works is straightforward once you know the mechanism: in metals, the same free electrons are responsible for both jobs. If a metal has a lot of mobile electrons that scatter infrequently, it conducts electricity well and also shuttles heat efficiently. If those electrons scatter a lot, both conductivities drop together. Modern experiments have confirmed that this proportionality holds even in structures as small as individual metallic nanowires. When researchers carefully removed the effects of contact resistance, the measured Lorenz number for a single-crystal silver nanowire matched the theoretically predicted value, confirming that the Wiedemann-Franz law still applies at the nanoscale.1PubMed Central. Effect of Electrical Contact Resistance on Measurement of Thermal Conductivity and Wiedemann-Franz Law for Individual Metallic Nanowires
The law is not perfect. It works best at room temperature and above, and it can break down in exotic situations, such as in certain strongly correlated electron systems or at very low temperatures where different scattering mechanisms dominate. But for everyday engineering and for understanding why your copper wiring also makes a good heat sink, it holds remarkably well.
Why Some Metals Conduct Better Than Others
If all metals have free electrons, why is copper a better conductor than iron, and silver better still? The answer comes down to how easily electrons move through each metal’s particular crystal lattice. Several factors control this.
- Number of free electrons: Metals like copper and silver each contribute roughly one free electron per atom, and those electrons happen to scatter very infrequently off the crystal lattice. Iron contributes electrons too, but its more complex electronic structure creates more scattering opportunities.
- Crystal structure: A highly ordered, defect-free lattice lets electrons glide through with fewer collisions. Metals with a face-centered cubic structure, like copper, silver, and gold, tend to have particularly smooth electron highways.
- Atomic mass and bonding stiffness: Heavier atoms or stiffer bonds change how lattice vibrations interact with electrons, altering the scattering rate.
Silver sits at the top for both thermal and electrical conductivity, but copper is close behind and far cheaper, which is why copper dominates real-world wiring and heat exchangers. Aluminum is lighter and cheaper still, so it shows up in high-voltage power lines and aircraft heat management, where weight matters more than squeezing out the last bit of conductivity.
What Happens to Conductivity as Temperature Changes
You might expect that heating a metal would make its electrons move faster and therefore conduct better. The reality is the opposite for electrical conductivity: as temperature rises, the atoms in the lattice vibrate more vigorously, and those vibrations scatter the free electrons more often. More scattering means more resistance, so a hot copper wire conducts electricity worse than a cold one. That is why the filament in an incandescent bulb has a much higher resistance when glowing white-hot than when it is off.
Thermal conductivity in metals is a bit more nuanced. At moderate and high temperatures, the electron contribution still dominates, but increasing lattice vibrations both scatter electrons and contribute a small amount of heat conduction on their own through lattice waves. The net effect varies by metal and temperature range, but in general, metals become slightly worse thermal conductors at very high temperatures.
The low-temperature end is more interesting. In very pure metals cooled close to absolute zero, lattice vibrations nearly vanish. Electron scattering drops dramatically, and both electrical and thermal conductivity can soar. For some amorphous (non-crystalline) transition metals, the behavior is more complicated. Research on electron-phonon interactions in these disordered metals has shown that their resistivity can actually decrease with increasing temperature in the low-temperature range before leveling off at higher temperatures, a reversal of the usual pattern that comes from the unusual way electrons scatter in a disordered atomic arrangement.2Low Temperature Physics. Electron-phonon interaction and electrical resistance of amorphous transition metals in the strong-coupling approximation
Alloys and Impurities Degrade Conductivity
Pure metals are the best conductors. The moment you start mixing in other elements to form an alloy, conductivity drops. Stainless steel, for instance, is mostly iron mixed with chromium, nickel, and other elements. It is a far worse conductor of both heat and electricity than pure iron, which is itself not a great conductor compared to copper or silver.
The reason is scattering again. Foreign atoms sitting in the crystal lattice act as obstacles. An electron cruising through a perfect copper lattice barely notices the regularly spaced copper atoms, but a zinc atom substituted into one of those sites creates a local disruption in the electrical potential. The electron scatters off it, losing momentum and direction. The more impurity atoms, the more scattering, and the higher the resistance.
This is why electrical wiring is made from very high-purity copper, typically 99.9% pure or better. Even small amounts of contamination noticeably reduce performance. On the other hand, alloys are often deliberately chosen for applications where you want a metal that resists heat transfer, like the stainless steel body of a thermos or the Nichrome wire in a toaster, which needs to resist current flow so it heats up instead of just passing the current through.
What Happens When Metals Get Very Small
In everyday-sized metal objects, free electrons scatter mostly off lattice vibrations and impurities. But when a metal wire or film shrinks to the nanoscale, a new scattering source takes over: the boundaries of the material itself. An electron in a copper nanowire only a few tens of nanometers across will hit the wire’s surface far more often than it hits a lattice vibration. Surface scattering becomes the dominant source of resistance.3Applied Physics Letters. Non-monotonic boundary resistivity for electron transport in metal nanowires
This matters enormously for modern electronics. The copper interconnects inside computer chips have shrunk to just a few nanometers wide, and at that scale, they conduct significantly worse than bulk copper. Chip designers now grapple with this as a major bottleneck: the transistors keep getting smaller, but the wires connecting them get resistive enough to slow signals and waste power as heat. Engineers have explored alternatives like cobalt or ruthenium for the narrowest wires, since the surface-scattering penalty differs by metal depending on how the electron wave functions interact with the boundary.
Despite the increased resistance, the Wiedemann-Franz law can still hold at the nanoscale, as shown by the silver nanowire experiments mentioned earlier. The electrons are still doing both jobs. They are just doing both jobs worse because the boundaries get in the way.
Proving That Electrons Are the Carriers
The idea that free electrons carry current in metals is so familiar now that it is easy to forget it once needed proving. In the early twentieth century, Richard Tolman and T. Dale Stewart performed an elegant experiment: they rapidly decelerated a coil of metal wire and measured the brief pulse of current that resulted. The logic was simple. If the charge carriers inside the metal have mass, they should keep moving forward for an instant when the wire suddenly stops, producing a measurable current. The direction of that current pulse would reveal whether the carriers were positive or negative.
Working with copper, aluminum, and silver, the experiments found that the carriers were negative and had a charge-to-mass ratio consistent with electrons. The measured values were slightly higher than the accepted electron value, between 10 and 20 percent too large, but there were signs of systematic error, and follow-up work by S.J. Barnett brought the numbers even closer to the expected value for ordinary electrons.4Proceedings of the Royal Society of London. Series A – Mathematical and Physical Sciences. The inertia of electrons in metals This was satisfying experimental confirmation of what the theory predicted: the current carriers in metals really are electrons, and they behave like free particles with essentially the standard electron mass.
When Heat Conducts Without Free Electrons
Metals are not the only good thermal conductors. Diamond, for instance, conducts heat several times better than copper, yet it is an electrical insulator with essentially zero free electrons. How? Diamond relies entirely on lattice vibrations, or phonons, to carry heat. Its carbon atoms are light and bonded extremely stiffly, so vibrations pass through the crystal at high speed with very little scattering.
First-principles calculations of diamond’s phonon transport have found thermal conductivities exceeding 2,600 W·m⁻¹·K⁻¹ in certain crystal orientations, far above the roughly 400 W·m⁻¹·K⁻¹ of copper.5PubMed Central. Thermal Transport Properties of Diamond Phonons by Electric Field The mechanism is entirely different from metals: it is the lattice itself doing the work, not electrons. This is why diamond finds use as a heat spreader in high-power electronics, pulling heat away from a laser diode or processor die without creating any unwanted electrical pathways.
The distinction matters for understanding what “good conductor” means. Metals are good conductors of both heat and electricity because the same carriers handle both. Diamond is a spectacular thermal conductor but a terrible electrical one, because its heat-carrying mechanism has nothing to do with mobile charges. When someone says a material is “a good conductor,” it is always worth asking: of what?
Metals in Extreme States
Metallic behavior is not limited to the elements you find on the periodic table’s familiar left and center blocks. Under enough pressure, even hydrogen, the lightest and most abundant element in the universe, can become a metal. Inside Jupiter and Saturn, hydrogen is squeezed to millions of times atmospheric pressure, forcing its electrons into a shared, delocalized state much like the free electrons in a copper wire.
Laboratory experiments have measured the electrical conductivity of this liquid metallic hydrogen at pressures around 1.4 to 1.7 million times atmospheric pressure. The measured static electrical conductivity falls in the range of 11,000 to 15,000 siemens per centimeter, and the data support models in which roughly 65% of the hydrogen molecules have dissociated into individual atoms, freeing their electrons to roam.6PubMed Central / PNAS. Conductivity and dissociation in liquid metallic hydrogen and implications for planetary interiors This metallic hydrogen is thought to be responsible for the powerful magnetic fields of gas giant planets, generated by convective currents of the conducting fluid deep within their interiors.
The hydrogen example underscores the core principle: metallic conduction is not about being a particular element. It is about having a population of electrons that are free to move. Squeeze any material hard enough to delocalize its electrons, and it will start conducting like a metal. The periodic table’s metals just happen to reach that state at everyday pressures and temperatures, which is what makes them so useful for wiring, cookware, and everything in between.
Common Misconceptions Worth Clearing Up
One persistent misunderstanding is that electrons race through a wire at the speed of light. They do not. The electrical signal travels near light speed, but the electrons themselves drift remarkably slowly, typically less than a millimeter per second in household wiring. Think of it like a long tube already packed with marbles: push one in at one end, and one pops out the other end almost instantly, even though no individual marble moved very far.
Another misconception is that metals conduct heat only through electrons. Lattice vibrations do contribute, just not very much compared to the electron contribution in most metals. In a few metals with unusual electronic structures, the lattice contribution is proportionally larger, but for copper, aluminum, and the other common engineering metals, electrons carry the vast majority of thermal energy.
A third common mistake is thinking that all metals are great conductors. Bismuth, manganese, and certain other metals have electrical resistivities many times higher than copper. Bismuth in particular is so poor an electrical conductor that it finds use in applications where you specifically want a metal that resists current flow. Its electronic structure, with a complex Fermi surface and a very small number of effective carriers, is more like a semimetal than a full-blooded conductor. The label “metal” guarantees some free electrons, but not necessarily a lot of them or ones that move easily.
Finally, people sometimes assume that superconductors are just very cold, very pure metals. Some are, like mercury and lead at temperatures near absolute zero. But many of the most practically useful superconductors are ceramics, materials that are normally insulators. Superconductivity involves a fundamentally different mechanism from ordinary metallic conduction: electrons pair up and travel through the lattice without any scattering at all, producing exactly zero electrical resistance. The free-electron picture that explains why your copper wiring works is not the same physics that makes a superconductor levitate above a magnet.