I have a new q,perhaps it may solve the problem,

suppose the electronic sturcture of a solid that represents a two dimensional square lattice of divalent atoms can be discribed using nearly free electron model with a weak potential such that U<<h^2/4M(Pi^2)*(Pi/a)^2

*=multiply

^2=square

where a=lattice parameter

is this solid a metal, an insulator or a semi conductor? why?