Question

A solid $AB$ has the $NaCl$ structure. If radius of cation $A^{+}$ is 120 pm, the maximum possible value of the radius of the anion $B^{-}$ is:

• A. 240 pm
• B. 60 pm
• C. 49.6 pm
• D. 290 pm

Answer 1

Answer: (D)

290 pm

For $NaCl$ structure with coordination number 6, we have radius ratio between $0.414-0.732$

Radius ratio (r) is $r_c/r_a$

$r_a=r_c/r$

For the maximum value of anion radius, we should have a minimum radius ratio $=0.414$

$r_a=120/0.414=289.855pm$