A crystal is a well-organized material : its perfect form is a result of the periodic repetition of an atom or a group of atoms.
Example of Halite or salt rock crystal
Chemical formula : NaCl
A salt rock crystal is made of sodium (Na+) and chlorine (Cl-) ions organized in spheres whose radii depend on the number of orbiting electrons.
When crystallisation occurs, these ions occupy particular positions : each Na+ ion is surrounded by six Cl- ions and, conversely, each Cl- ion is flanked by six Na+ ions. The resulting structure is a squared figure which, repeated indefinitely, represents the crystal. A unit of repetition of the crystalline material is called an “elementary network”.
The elementary network of salt rock
Only X-rays can show the existence of the elementary network, as its dimensions are measured in Angströms (1 angström = 0,000 000 01 centimeter).
Thus, a 5mm ridge salt rock crystal would contain 10 000 000 elementary cubes.