Question

The ionic radii of $R{b}^{+}$ and $I^{-}$ are $1.46$ and $2.16\stackrel{o}{A}.$ The coordination number for the cation is :

• A. 2
• B. 4
• C. 6
• D. 8

Answer 1

The correct option is C 6

$\text{Ionic radii of}R{b}^{+},r_{R{b}^{+}}=1.46\stackrel{o}{A}$
$\text{Ionic radii of}{I}^{-},r_{I^{-}}=2.16\stackrel{o}{A}$
$\Rightarrow \frac{r_{R{b}^{+}}}{r_{I^{-}}}=\frac{1.46}{2.16}=0.67$
Radius ratio for $RbI$ is 0.67
Since,
$0.414<0.67<0.77$
Octahedral void is occupied.
Thus, the coordination number is 6.
Rock salt structure compounds have the coordination number of 6.