At low temperatures, atomic motion is limited to rotation and vibration, and the immobilized atoms or molecules bond tightly to each other in solid, closely packed, well-defined structures. Such structures define the solid phase. In a crystalline solid—salt, sugar, tempered chocolate—the particles are arranged in a regular, repeating array, while in amorphous solids—boiled candies, glass—they are randomly oriented. Large, irregular molecules like proteins and starch often form both highly ordered, crystalline regions and disordered amorphous regions in the same chunk of material. Ionic bonds, hydrogen bonds, and van der Waals bonds may be involved in holding the particles of a solid together.