Why is mercury a metal?

Mercury is a metal because is has free electrons as do all metals. As a result it is a good conductor of electricity as are all metals. All metals form compounds by giving up those free electrons to become positive ions. Mercury gives up two electrons to become an ion of charge plus two. read more

Mercury also fulfills the other conditions - "a metal (Greek: Metallo, Μέταλλο) is a chemical element whose atoms readily lose electrons to form positive ions (cations), and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms". read more