Sn, C, Si and Ge are all group XIV elements . Yet , Sn is a conductor , C is an insulator while Si and Ge are semiconductors . Why?

Explanation

A marterial is a conductor if in its energy band diagram, there is no energy gap between conduction band and valence band. For insulator, the energy gap is large and semiconductor the energy gap is moderate.
The energy gap for Sn is `0 eV`, for C is `5.4 eV`, for Si is `1.1 eV` and for Ge is `0.7 eV`, related their atomic size. Therefore Sn is a conductor, C is an insulator and Ge and Si semiconductors.