Copper conducts heat well because its atoms contain only one free valence electrons in the outer shell. Elements with a low number of valence electrons transfer heat the best, and copper has only one.
Valence electrons move about freely. Just as the impact of a billiard ball striking another billiard ball is greater than when it collides with multiple balls, a single electron imparts significant energy to other electrons. Since copper has only one electron in its outer shell, it transfers a strong repelling energy to electrons with which it collides.
It is the degree of energy within the metal that imparts heat. Since heat increases when the subatomic particles are able to move without resistance throughout the metal, copper is a good conductor of heat. If heat is applied to one end of a cooper wire, it travels quickly to the other end, as the electrons move unimpeded by resistance.
Electrical conductivity is indirectly proportional to resistance a metal has when an electrical field is applied. Conduction is proportional to the current. Copper has a low resistivity measured in ohms. Its conductivity, measured by siemens per meter, is high. Copper is second only to silver in its conductive properties.