The one word that completes the sentence "Metals are the best electrical conductors because they have free ________" is "electrons." Metals conduct electricity so well because they have free electrons that are not tightly bound to atoms and can move freely throughout the metal lattice. This "sea of electrons" allows electric current to flow with minimal resistance. The free electrons in metals enable effective conduction of electric charge, which is why metals like silver, copper, and gold are excellent conductors.