A binary solid () perfectly crystallizes in a rock salt () structure. If the unit cell edge length i — Solid State Chemistry Question

💡 Solution & Explanation

In a rock salt (FCC) structure, the cations and anions touch along the cell edge. Thus, the edge length $a = 2(r_+ + r_-)$. Given $a = 400\text{ pm}$ and $r_+ = 75\text{ pm}$, we have $400 = 2(75 + r_-) \implies 200 = 75 + r_- \implies r_- = 125\text{ pm}$.