P$.4$

$($a$)$ Derive planar density expressions for FCC

$(100)$ and $(111)$ planes in terms of the atomic

radius $R.$

P$.5$

Derive the planar density for:

a$)BCC(\frac{1}{b}ar(3)1)$

b$)$ FCC $(220)$

c$)FCC(213)$

P$.6$

Determine indices for the directions shown in the following hexagonal unit cells:

P$.7$

Calcul ate the radius of an iridium atom, given that Ir has an FCC crystal structure, a density of

$22\mathrm{.}4\frac{g}{c}{m}^3,$ and an atomic weight of $192\mathrm{.}2\frac{g}{m}ol.$