The radius of${\mathrm{Ag}}^{+}$ ion is 126 pm and that of I- ion 216 pm. The coordination number of  ${\mathrm{Ag}}^{+}$ion is :

$\frac{{\mathrm{r}}^{+}}{{\mathrm{r}}^{-}}=\frac{{\mathrm{r}}_{{\mathrm{Ag}}^{+}}}{{\mathrm{r}}_{{\mathrm{I}}^{-}}}=\frac{126}{216}=0.5834$

If $\frac{r^{+}}{r^{-}}\in [0.414,0.732)$.

6 is the coordination number. The structure of compound crystals must resemble that of rock salt.