What characteristic describes a good insulator?

A good insulator is characterized by having few free electrons. This trait is critical because materials with a low number of free electrons do not allow electric charges to move freely. When there are fewer free electrons, it significantly reduces the material's ability to conduct electricity and heat, making it an effective insulator.

In the context of insulating materials, the presence of few free electrons ensures that they can effectively resist the flow of electrical current. This resistance is essential for protecting against electrical hazards, maintaining safety standards, and improving energy efficiency in a variety of applications. Therefore, this characteristic defines the insulating properties of materials like rubber, glass, and ceramics, which serve to prevent the transfer of heat or electricity.