In a crystal, the particles are arranged in a regular pattern that repeats in every direction. Salt, ice and quartz are crystals. In an amorphous solid such as glass, the particles have no regular arrangement.
The difference shows up on heating. The particles of a crystal are all held in the same way, so the crystal melts at one temperature, and the temperature stays at this melting point until all the solid has melted.
In glass, the particles are held more strongly in some places than in others, so glass has no sharp melting point. It softens gradually over a range of temperatures, which is why hot glass can be bent and blown into shape.
Crystals can be sorted by the particles they are made of and the forces that hold those particles together. In an ionic crystal such as sodium chloride, positive and negative ions are held together by ionic bonds.
In a molecular crystal such as dry ice, molecules are held together by intermolecular forces. In a covalent network crystal such as diamond, every atom is joined to its neighbours by covalent bonds, so the whole crystal is one giant structure.
In a metallic crystal such as copper, metal ions are held together by free electrons that move through the whole crystal. This kind of bonding is called metallic bonding.
The elements in a substance usually show which kind of crystal it forms. A compound of a metal with a nonmetal, such as sodium chloride, forms an ionic crystal, and a metal or an alloy forms a metallic crystal.
Substances made only of nonmetals usually form molecular crystals. Only a few, such as diamond, graphite, silicon, silicon dioxide, \mathrm{SiO_2}, and silicon carbide, \mathrm{SiC}, form covalent network crystals.
There are exceptions. Ammonium chloride, \mathrm{NH_4Cl}, contains no metal element, but it is an ionic crystal made of ammonium ions, \mathrm{NH_4^+}, and chloride ions, \mathrm{Cl^-}.
To melt a crystal, the forces between its particles must be overcome. Molecular crystals are held only by weak intermolecular forces, so they melt at low temperatures, and some, such as dry ice and iodine, sublime easily.
Ionic crystals melt at high temperatures, from 801 °C for sodium chloride to about 2800 °C for magnesium oxide. Covalent network crystals also need high temperatures, because covalent bonds must break: silicon melts at 1414 °C, and graphite turns into a gas at about 3600 °C.
Metallic crystals vary the most, from mercury, which is a liquid at room temperature and freezes at -39 °C, to tungsten, which melts at 3422 °C.
Ionic crystals are hard but brittle. They do not conduct electricity as solids, but they do when molten or dissolved in water, because the ions are then free to move.
Molecular crystals are soft and do not conduct electricity. Covalent network crystals such as diamond are very hard and do not conduct either, except graphite, in which one electron from each carbon atom can move along the layers.
Metallic crystals conduct electricity and heat well because of their free electrons. They can be hammered into sheets and drawn into wires, since the free electrons keep holding the ions together as the layers slide.