1)  Construction/Atomic Arrangement –

Diamond is made up of carbon atoms. Carbon is tetravalent.

∴ Each carbon atom forms 4 covalent bonds.

Diamond cubic structure is combination of two interpenetrating FCC sub-lattices along the body diagonal by ¼th  cube edge. One sub-lattice has its origin at (0,0,0) & other sub-lattices has its origin 1/4 of way along body diagonal i.e. at point (a/4, a/4, a/4). The basic diamond lattice and the atomic position of cubic cell of diamond are as shown in above fig.

The fractions e.g. $\dfrac{3}{4},\dfrac{1}{2},\dfrac{1}{4}$ denote the height above the base in units of cube edge. The atomic basis is 2 atoms/lattice point. One atom at (0,0,0) & the other at (a/4, a/4, a/4) as stated above.

4)  Atomic Packing Factor (APF) –

APF=$ \dfrac {(Number\ of\ atoms/unit\ cell) \times (Volume\ of\ 1\ atoms)}{Volume\ of\ unit\ cell} \\$ $=\dfrac {8\times \dfrac {4}{3} \Pi r^{3}}{a^{3}}\$

$=8\times \dfrac {4}{3} \Pi \times \left (\dfrac {\sqrt {3}}{8}a \right )^{3} =0.34\$

∴ It is loosely packed structure but covalent bonds being the strongest.

∴ It is hard.