Question

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

Answer 1

$\text{Ionic radii of}R{b}^{+},r_{R{b}^{+}}=1.46\stackrel{\circ }{\text{A}}$
$\text{Ionic radii of}{I}^{-},r_{I^{-}}=2.16\stackrel{\circ }{\text{A}}$
$\Rightarrow \frac{r_{R{b}^{+}}}{r_{I^{-}}}=\frac{1.46}{2.16}=0.67$
Radius ratio for $RbI$ is $0.67$
Since,
$0.414<0.67<0.73$
Octahedral void is occupied.
Thus, the coordination number is $6$.
Rock salt structure compounds have the coordination number of $6$.