insulator's atomic structure does not have enough free electrons, and free electrons are what conducts electricity.
because conductors have free electrons, they conduct electricity
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Current is the movement of charges in a conductor. Materials that allow many electrons to move freely are called conductors.

Electrons are tightly bound to their atoms in materials like wood, glass, plastic, air and cotton. Since the electrons can't move, they don't conduct electricity very well. These materials that don't conduct electricity well are called Insulators.