Try this: in your home, touch something made of wood, and then touch something metal nearby. The metal probably feels cooler. But why is this? Is there some sort of magical property of metal that actually makes it colder than other objects?
Well, no. If you are at all familiar with thermodynamics, you probably understand that objects tend towards thermodynamic equilibrium with their surrounding environment; in other words, if an object is placed in a room of a different temperature, the object and the room will eventually become the same temperature. Even if you have no formal understanding of thermodynamics, you likely have an intuitive sense for this phenomenon, as you’ve probably forgotten about a warm beverage you’ve left out, finding it hours later to be unpleasantly lukewarm.
So if objects tend towards the same temperature as their environment, then why does metal feel so much colder than other materials? The answer lies in the high thermal conductivity of metals. When you touch metal that is at room temperature, the metal conducts heat energy away from your hand—which hopefully is around 98.6 F (37 C)—faster than most other materials, making it feel colder. Likewise, a material that is colder than room temperature will heat up faster when placed on room temperature metal than on a less conductive material. If you place ice cubes on surfaces of metal and wood that are both at room temperature, the ice cube on the metal surface will melt faster, as the metal conducts heat more efficiently to the ice cube, warming it more quickly.
But why do metals have such high thermal conductivity? The answer most commonly given is that solid metals have free valence electrons that move independently of their individual metal atoms, allowing metals to carry charge and conduct heat and electricity. However, this idea is a bit of an oversimplification.
The reason why metals conduct heat well is slightly more complex and is better understood in the context of band theory. Band theory explains these properties by postulating that the position of electrons in a solid can be described by discrete “bands” or energy levels. While an electron in free space, or one not bound to a solid, can exist in any specified energy level, electrons bound by solids only exist in certain energy levels called allowed bands and are not found in so-called “forbidden bands.” Metals make such good conductors because the outermost bands—those that carry the most energy—are allowed bands, allowing electrons in these solids to become more energized and conduct heat and electricity more efficiently. Moreover, these outermost “conductive bands” are easily obtainable for electrons in metals, as there is no gap between the conductive bands and the normal valence bands in which the valence (outermost) electrons exist.
Likewise, the properties of insulators and the coveted semiconductors can be explained by band theory. Insulators have large gaps of forbidden bands, making it very difficult to excite electrons to the higher energy levels of conductive bands. Contrarily, semiconductors have very small gaps of forbidden bands, allowing their electrons to exist in both low and high energy states. This makes semiconductors very useful, as they can act as both insulators and conductors depending on the temperature of their environment. At high temperatures, the more excited electrons allow these materials to act like conductors, and at low temperatures, the consequential drop in electron energy cause semiconductors to act more like insulators.
So band theory can explain the conductive properties of many materials, but what does it have to do with our original question? Well, because metals have the outermost electron bands allowed, they are able to conduct heat and electricity more efficiently than other materials. Therefore, when compared with an insulating material, whose outermost bands are likely forbidden or exist too far away from other bands to be easily reachable, metal will feel cooler, as it conducts heat away from your hand with greater efficiency.
References:https://docs.google.com/document/d/1bDNuMsNZwbV5r1SJq0EYjrRuf4gClksdlHxy2MrzPQE/edit?usp=sharing