Now, imagine adding other close-by phosphorus atoms. Each frees its extra electron to wander. However, the wandering electrons are now attracted by any of the positively ionized phosphorus atoms.
The net result is that electrons (and electrical current) can move freely though the volume where the phosphorus atoms reside. In fact they move so quickly that it is more appropriate to represent them as a blurred blue cloud.
This end of the crystal has thus been converted to an electrical conductor.
