Find the total volume of a unit cell given the radius of the atom.
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The edge length \( a \) is related to the atomic radius \( r \) and depends on the type of cubic unit cell. Use \( a = 2r \) for simple cubic, \( a = \frac{4r}{\sqrt{3}} \) for BCC, and \( a = \frac{4r}{\sqrt{2}} \) for FCC.