Which method is used to determine the edge length of a unit cell?

What is the relationship between the edge length $(a)$ and the radius of the atom $(r)$ in a simple cubic unit cell?

In an orthorhombic crystal system,the values of $a$,$b$,and $c$ are $4.2 \, \mathring{A}$,$8.6 \, \mathring{A}$,and $8.3 \, \mathring{A}$ respectively. Given the molar mass of the solute is $155 \, g \, mol^{-1}$ and the density is $3.3 \, g \, cm^{-3}$,calculate the number of formula units $(Z)$ per unit cell.

Copper crystallises in $fcc$ with a unit cell edge length of $361 \ pm$. What is the radius of the copper atom in $pm$?

An element has a body-centred cubic $(bcc)$ structure with a cell edge of $288 \, pm$. The density of the element is $7.2 \, g/cm^{3}$. How many atoms are present in $208 \, g$ of the element?

Iron exhibits $bcc$ structure at room temperature. Above $500^{\circ} C$,it transforms to $fcc$ structure. Find the ratio of the density of iron at room temperature to that at $500^{\circ} C$. (Assume the atomic radii and the molar mass of iron remain constant even with variation in temperature)