Question

Pick out the correct statement(s) for $CsCl$ crystal:

• A. Co-ordination number for $C{s}^{+}$ and $C{l}^{-}$ is $8$.
• B. $\frac{r_{C{s}^{+}}}{r_{C{l}^{-}}}=0.732$
• C. The structure changes to $NaCl$ structure at $760$ K.
• D. $C{l}^{-}$ ions are present at cubic sites.

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A Co-ordination number for $C{s}^{+}$ and $C{l}^{-}$ is $8$.
B $\frac{r_{C{s}^{+}}}{r_{C{l}^{-}}}=0.732$
C The structure changes to $NaCl$ structure at $760$ K.
D $C{l}^{-}$ ions are present at cubic sites.

Limiting radius ratio	Co-ordination number	Shape	Examples
$<0.155$	$2$	Linear	$Be{F}_2$
$0.155-0.225$	$3$	Trigonal planar	$B_2{O}_3$
$0.225-0.414$	$4$	Tetrahedral	$ZnS,Si{O}_4^{4-}$
$0.414-0.732$	$4$	Square planar	$PtC{l}_4^{2-}$
$0.414-0.732$	$6$	Octahedral	$NaCl$
$0.732-0.999$	$8$	Body central cubic	$CsCl$

Cesium Chloride (CsCl) type structure: CsCl has body-centred cubic (bcc) arrangement. Each $C{s}^{+}$ ion is surrounded by $8$ $C{l}^{-}$ ions and each $C{l}^{-}$ ion is surrounded by $8$ $C{s}^{+}$ ions i.e. this structure has $8:8$ co-ordination. A unit cell of CsClconsists of only one unit of CsCl i.e. One $C{s}^{+}$ ion and one $C{l}^{-}$ ion. Few examples of compounds having $CsCl$ structure are CsBr, Csl, CsCN, TICI, TIBr, TII and TICN.

CsCl structure on heating to about $760$ K, changes to NaCl structure.