What is an essential characteristic of a good electrical conductor?

An essential characteristic of a good electrical conductor is the presence of free electrons. In materials such as metals, the atomic structure allows for some electrons to become delocalized and move freely. This mobility of free electrons is what enables electric current to flow easily through the material, thereby leading to low resistance.

Good conductors, like copper and aluminum, have a high density of these free electrons, which facilitates the transfer of electrical charge with minimal energy loss. In contrast, materials with high resistance do not allow for easy electron movement, and those with low thermal conductivity do not efficiently transfer heat, which is not directly related to electrical conductivity. High density may contribute to conductivity but is not the pivotal characteristic; without free electrons, no effective conduction would take place. This makes the presence of free electrons the defining feature of a good electrical conductor.