Question

In alloys, like constantan, atoms are in disorder so alloys have big resistivity. justify the statement.

Answer 1

The Metals, including alloys, have free electrons as charge carriers. Their movement controls by defects. One of defects of usual metal is oscillations of atoms due to temperature. More temperature - more oscillations - more collisions of electrons with atoms - less mobility - more resistivity. In alloys, like constantan, atoms are in disorder so alloys have big resistivity. Their additional disorder due to temperature increase is insignificant. That is why alloys have no temperature dependence of resistivity. The same picture is with metal glasses.

Answer 2

a disorder alloy has short range order. hence the mobility of charge carriers decreases by several orders of magnitude. (comparing to their crysralline counterpart). This makes the conductivity low (very low) in disordered alloys. Well we know that low conductivity means high resistivity Since short range order is not much different , the band gap does not change drastically in disordered alloys .Slightly higher band gap gives in disorders alloys may also increase resistivity but the mobility effect is less i.e. more or big resistivity 
HOPE THIS HELPS U
:-)