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. 

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where it is written.......

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CsCl crystallizes in a bcc structure. With the Cl atoms at the corner and Cs atom in the centre. Hence, number of Cs atoms = 1 and number of Cl atoms = 1/8 x 8 = 1
Therefore, z = 2

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  ya he knows that here z=2...but he was asking natasha why we have taken...z=1....

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 its written in pradeep.if u have then please check it on page 1/33 in the box titled FORMULA USED.

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 sorry varun i dont have pradeep....but soon i will get.....then may be i m able to tell u....

but it can be the printing mistake......also...

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Neeshu, yes, must be a printing mistake. 

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 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.

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ya @varun may be the reason which u r telling is right..but wait for experts answer..hope they will gave soon..but they are not giving my answer....they r imp for me..pls any of u try atleast..pls...

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