What property makes substances classified as good conductors?

Good conductors are characterized by the free movement of electrons. In metals, which are commonly good conductors, the outer electrons are not tightly bound to their respective atoms. This allows these electrons to move freely throughout the material, enabling the efficient transfer of heat and electrical energy. As a result, substances with this property can easily conduct electricity and heat, which is why elements like copper and aluminum are widely used in electrical wiring and thermal applications.

In contrast, properties like high thermal resistance, low melting point, or high density do not directly contribute to conductivity. High thermal resistance implies that a material does not conduct heat well, low melting point does not relate to electron mobility, and high density does not inherently indicate the ability to conduct electricity or heat effectively. Thus, the ability for electrons to move freely is the definitive property that classifies a substance as a good conductor.