Why is pure silicon a poor conductor at room temperature? a. its valence electrons are held tightly within a crystal structure. b. it is a metal. c. it is a nonmetal. d. its valence electrons are held loosely by a positive nucleus.

The appropriate answer is a. it's valence electrons are held tightly within the crystal structure. Metals are good conductors because they share electrons and so they are free to move about enabling it to be a good conductor. Any substance that has electrons tightly held within bonds will not be good conductors. If the structure of pure silicon had its valence electrons held loosely by a positive nucleus then it would be able to conduct.