Solid State - Result Question 28
28. Which of the following statement(s) is/are correct?
(1998, 2M)
(a) The coordination number of each type of ion in $CsCl$ crystal is $8$
(b) A metal that crystallises in bcc structure has a coordination number of $12$
(c) A unit cell of an ionic crystal shares some of its ions with other unit cells
(d) The length of the unit cell in $NaCl$ is $552 \hspace{2mm} pm$.
$\left(r _{Na^{+}}=95 \hspace{2mm} pm ; r _{Cl^{-}}=181 \hspace{2mm} pm\right)$
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Answer:
Correct Answer: 28. (a, c, d)
Solution:
- (a) The unit cell of $CsCl$ has bcc arrangement of ions in which each ion has eight oppositely charged ions around it in the nearest neighbours as shown below :
(b) In bcc, coordination number of atom is $8$ .
(c) In an unit cell, a corner is shared in eight unit cells and a face centre is shared between two adjacent unit cells.
(d) In $NaCl$ unit cell; $2\left(r _{Na^{+}}+r _{Cl^{-}}\right)=a$
$ \Rightarrow \quad a=2(95+181)=552 \hspace{2mm} pm $
Hence, $a, c, d$ are correct.
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