Question

The minimum anion-to-cation radius ratio at which a 3-fold coordination becomes possible is ............. [round off to 2 decimal places]

Hint:

Radius ratio rules help predict coordination: - CN = 2 → 0.00–0.15 - CN = 3 → 0.15–0.22 - CN = 4 → 0.22–0.41, etc.

Answer 1

Step 1: Recall radius ratio rule.
Radius ratio = $\dfrac{r_c}{r_a}$, where $r_c$ = cation radius, $r_a$ = anion radius.

Step 2: Threshold for 3-fold coordination (triangular planar).
For coordination number = 3, minimum radius ratio = 0.155.

Step 3: Round off.
\[ 0.155 \approx 0.15 \, \text{(2 decimal places)} \]

Final Answer: \[ \boxed{0.15} \]