Kelly- The reason metals conduct heat so well is that the electrons in them can move around easily, and they can carry heat from one part to another. (Those are the same electrons that carry electricity around, making metals good electrical conductors.) If you search this site, youíll find other answers discussing different types of heat transfer, including heat conduction by non-metals.
Most ordinary metals have about the same density of electrons (number per volume), so the main reason for the differences in how well they conduct electricity is the difference in how easily the electrons move around. In some metals (copper, gold, silver, aluminum) the electrons can travel past hundreds of atoms before bumping into something that makes them change directions. In others (especially alloys with more than one type of metal atom) the electrons travel only past a few atoms before bouncing off some irregularity in the material. This is not the only factor giving the different thermal conductivities, but it is probably the biggest one.
The same things that determine how well those electrons conduct heat also determine how well they conduct electricity. Thatís why the ratio of the thermal conductivity to the electrical conductivity comes out very similar for most metals near room temperature.
(republished on 07/27/06)