University physics note reads:
1) Electrical conductors are materials in which some of the electrons are free electrons. Free electrons are not bound to the atoms.
2) Electrical insulators are materials in which all of the electrons are bound to atoms.
But I am not quite understand that why free electrons are not bounded to the atoms in metal. According to my understanding in the word 'bounded to xxx', it means something is free from the influence of something, and in this case free electrons are free from the attraction of the atoms and have enough energy to escape from its surface.