Why noble gas have comparatively larger in atomic size ?

Questions :Why noble gas have comparatively larger in atomic size ?

Because they have van der Waal's which are expected to have larger magnitude where as the other member of period have either covalent or metallic radii which are the less in magnitude .

Questions : Why znS  show frenkel defects and AgBr  show which defect   

Main exception  is size of Ions  znS show frenkel defects due to its Ions have  large difference in size

AgBr show both frenkel defects and Schottky defects .

Questions : Why Ionic solid conduct the electricity in the molten state but not in a solid state ?

In the ionic solid , electricity conduct due to the movement of of Ions, Other hand  in the solid state , the Ions can't move anymore and remain held strong together by strong electrostatic force of attraction . This is the reason   they  work like a insulator .

Questions: What is the significance of lattice points ?

The lattice points represents the position of particular constitute (atoms,Ions, or molecules ,) in a crystal lattice .the arrangement of lattice points in space is responsible for the shapes of size of particular Crystal solid .