it is written in book that while calculating density, z=1 for ionic compounds such as CsCl forming BCC structures.but in bcc structures no of atoms =2 then why have we taken Z=1
You yourself answered the question correctly. In case of ionic compounds, we consider the number of formula units per unit cell. Thus, Z will denote the number of formula units per unit cell and not the number of individual ions. Since CsCl has BCC structure, therefore Z = 1.
Another important point to note is that for ionic compounds, we use the term formula units and not molecules. Molecules are formed by sharing of atoms and not by transfer of electrons.
hey friends i got some clue after going deeply through the chapter.i think when we talk about lattice formed by atoms we take z=2 but when we talk about ions we take z=no of formula units ia a unit cell i.e no of molecules in one unit cell.since in a unit cell having bcc structure, there is one Cs atom at the centre and one Cl atom (1/8 * 8) thus number of molecules in one unit cell=1.that might be the reason for taking z=1 for bcc structures formed by ionic compounds.but i am not sure so lets wait for the expert's answer.