Problem #$4$: $(5)$

Strontium $($ Sr $)$ has an FCC crystal structure, an atomic radius of $0\mathrm{.}215$ nm $,$ and an atomic weight

of $87\mathrm{.}62\frac{g}{m}ol.$ Calculate the theoretical density for Sr $.$

Problem #$5$: $(5)$

Explain the difference between the structure crystalline: BCC$,$ FCC$,$ and HCP$.$