Which physical property explains the difference in ability of copper and rubber to transmit electricity?

The copper is a conductor and the rubber is an insulator. Conductors allow electricity to be transmitted freely while insulators prevent most of the current from transmitting. The outer electrons of the conductor's atoms are not very stable and are able to freely move around the material. In a material such as copper, the valence electrons are essentially free and strongly repel each other. The atoms of insulators such as rubber, hold tight to their outer electrons which prevents electricity from flowing freely through.