This long-range order builds visible crystals with flat faces that meet at specific angles. The highly organized structure gives crystalline solids a very sharp melting point. They change from solid to liquid at one exact temperature.
This happens because all the identical bonds in the lattice break at the same time. In contrast, amorphous solids like plastics soften slowly over a wide temperature range. The repeating crystal lattice also scatters X-rays in a very distinct pattern.
Scientists use a technique called X-ray crystallography to map these patterns. This lets them figure out the exact positions of atoms inside the solid. Students often think all crystals must be clear and shiny like diamonds.
However, many dull or powdery everyday substances are actually made of thousands of tiny crystals. A solid forms a crystal when its liquid cools down slowly. The slow cooling gives atoms plenty of time to pack into the most stable pattern.
Chemists group crystalline materials by the geometric shape of their smallest repeating unit. These shapes include simple forms like cubic, hexagonal, and tetragonal building blocks.