Crystalline and amorphous solids
Solids are of two kinds. In crystalline solids the atoms or molecules sit in a regularly repeating order forming a crystal lattice; the particles vibrate about the lattice nodes but do not leave them. In amorphous solids (glass, wax, resin, plastic) the particles are arranged randomly; they resemble a very thick liquid. A monocrystal is a body whose whole piece is a single lattice (quartz, diamond, a grain of table salt); its properties depend on direction, which is called anisotropy: for example, mica splits into thin layers in only one direction. A polycrystal consists of very many tiny crystals pointing in random directions (metals, rock), so as a whole it is isotropic, i.e. its properties are the same in all directions. Amorphous solids are also isotropic and have no definite melting temperature; one element can form different lattices – graphite and diamond are both carbon, yet their properties are very different.
Observation (with an adult): look at a grain of table salt or sugar through a magnifier and sketch its crystal shape. Compare it with a piece of glass. How do the edges of broken glass and of a salt crystal differ? Do not touch sharp glass; work only with safe samples prepared by an adult.