A solid in which electrons are able to move about easily from one atom to another is called a conductor.

Here's why:

* Conductors allow the flow of electric current. This is because their electrons are loosely bound to their atoms and can move freely throughout the material. Metals are excellent examples of conductors.

* Insulators, on the other hand, have electrons tightly bound to their atoms. This makes it very difficult for electrons to flow, and they don't conduct electricity well.

Let me know if you'd like more information about conductors or insulators!