Question

Assertion :$CsCl$ has body-centered cubic arrangement. Reason: $CsCl$ has one $C{s}^{+}$ ion and $8C{l}^{-}$ ions in its unit cell.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is C Assertion is correct but Reason is incorrect

$CsCl$ has body-centered cubic arrangement. In bcc, atoms are present at corners and at the body center. Now, $C{s}^{+}$ is present at the body center while $C{l}^{-}$ ions are present at the corners. So,

Number of $C{l}^{-}$ ions $=8$ (Since there are $8$ corners in a cube)

Number of $C{s}^{+}$ ion $=1$

But, the contribution of atoms/ions at the corner is $\frac{1}{2}$ to the unit cell and contribution of an ion at the body center is $l$ to the unit cell.

So, effective number of $C{l}^{-}$ ion in unit cell $=\frac{1}{8}\times 8=1$

Effective number of $C{s}^{+}$ ion in unit cell $=1$