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

Copper (atomic mass $= 63.5$) crystallises in a $fcc$ lattice and has density $8.93 \, g \, cm^{-3}$. The radius of copper atom is closest to $.... \, pm$.

$A$ compound has $fcc$ structure. If the density of the unit cell is $3.4 \text{ g cm}^{-3}$,what is the edge length of the unit cell? (Molar mass $= 98.99 \text{ g mol}^{-1}$)

Silver forms $ccp$ lattice and $X$-ray studies of its crystals show that the edge length of its unit cell is $408.6 \,pm$. Calculate the density of silver ( Atomic mass $= 107.9 \,u$ ).

Select the $INCORRECT$ option regarding the cubic crystal system-

$Na$ metal crystallizes in a $BCC$ structure. If the edge length of the unit cell is $4.29 \ \overset{o}{A}$,then the radius of the $Na$ atom is = ...... $cm$.