Read the Assertion and Reason below and choose the correct option. — Solid State Chemistry Question

💡 Solution & Explanation

Density depends on the specific molar mass and atomic radius of the element ($d = \frac{Z \cdot M}{N_A \cdot a^3}$), so it is not fixed for a given unit cell type. However, the packing fraction is a strictly geometric property (e.g., 74% for FCC) and remains constant for all atomic solids crystallizing in that specific unit cell arrangement. Therefore, correct answer is D.