In an ionic solid anions are arranged in hcp array and cations occupy $\frac{2}{3}$ of octahedral voids.

What is the formula of ionic compound? [consider $\mathrm{A}=$ cation; $\mathrm{B}=$ anion]

Calculate the number of unit cells in $1 \mathrm{~cm}^3$ of an element if unit cell edge length is $2.0 \times 10^{-8} \mathrm{~cm}$.

Calculate the radius of an atom of an element in pm if it forms bcc unit cell structure having edge length $4.3 \times 10^{-8} \mathrm{~cm}$.

The attractive interactions between cations and mobile electrons in metallic solid is called