Which property makes metals effective electrical conductors?

Metals are effective electrical conductors primarily due to the presence of free electrons. In metallic bonding, atoms share their outer electrons in a "sea of electrons," which allows these electrons to move freely throughout the metal lattice. This mobility of electrons is what enables electrical current to flow through metals very easily when a voltage is applied. The abundance of these free-moving electrons is a characteristic feature of metals that contributes significantly to their conductivity.

While the crystalline structure of metals can influence other properties, it is not the key factor in electrical conductivity. The low density and high melting point of metals are also important properties, but they do not directly relate to the ability of metals to conduct electricity. Ultimately, the free electrons are the critical feature that makes metals excellent conductors of electricity.