C, Si and Ge have same lattice structure . Why is C an insulator while Si and Ge are intrinsic semiconductors ?

Explanation

The 4 bonding electrons of C , Si or Ge lie respectively , in the second , third and fourth orbit . Hence , energy required to take out an electron from these atoms (i.e., ion ionisation energy `E_(g)`) will be least for Ge , followed by Si and highest for C . Hence number of free electrons for conduction in Ge and Si are significant but negligibly small for C .