In crystalline solids, atoms or molecules are arranged in regular and long range order fashion in three dimensional pattern. These have sharp melting point, flat faces, sharp edges, bounded by well defined planes. A large number of unit cells, each of which possess a definite geometry bounded by plane faces give rise to the formation of crystal. A point at the corner of unit cell contributes for $1/8$ of each such point of unit cell. A point along the edge contributes for $1/4$ of each such point to unit cell. A body centred point contributes for 1 of such point of unit cell. Coordination number is the number of neighbours that each ion is surrounded by an oppositely charged ions. Radius of the unit cell in sc, fcc and bcc is $\frac{\mathrm{a}}{2}, \frac{\mathrm{a}}{2\sqrt{2}} \mathrm{and} \frac{\sqrt{3}{\displaystyle \mathrm{a}}}{4}$where $\mathrm{a}$ is the edge length of the unit cell.

An alloy of $\mathrm{Cu}$ and gold crystallizes in cubic lattice in which the gold atoms occupy the lattice points at the corners of cube and copper atoms occupy the center of each face. The formula of this compound is

In solid ${\mathrm{NH}}_3$, each ${\mathrm{NH}}_3$ molecule has six other ${\mathrm{NH}}_3$ molecules as nearest neighbours. $\mathrm{\Delta H}$ of sublimation of ${\mathrm{NH}}_3$ at the melting point is $30.8 \mathrm{kJ} {\mathrm{mol}}^{-1}$ and the estimated $\mathrm{\Delta H}$ of sublimation in the absence of $\mathrm{H}$-bonding is $14.3 \mathrm{kJ} {\mathrm{mol}}^{-1}$. The strength of $\mathrm{H}$-bond in ${\mathrm{NH}}_3$ ion in $\mathrm{kJ} {\mathrm{mol}}^{-1}$ is: