Crystalline solids consist of atoms, ions and molecules arranged in definite and repeating three-dimensional patterns. Unlike amorphous solids that melt at a range of temperatures, crystalline solids have definite melting points. Crystalline solids include metallic, ionic, network atomic and molecular solids, and true solids are crystalline.
In metallic solids, valence electrons hold together the positively charged nuclei of metal atoms. The electrons are delocalized because they aren't bound to specific atoms. Metallic solids, including most metals and their alloys, are hard, shiny and ductile and have high electrical and thermal conductivity. Electrostatic attraction bonds anions and cations to make a crystal lattice and form an ionic solid, such as sodium chloride or table salt. Each ion in an ionic crystal is encircled by ions with an opposite charge.
In network atomic solids, covalent bonds hold atoms together to form large crystals. These solids include diamonds, rubies and other gemstones. Molecular solids, such as ice, consist of covalent molecules held together by intermolecular forces. These bonds are not as strong as those found in other solids, and molecular solids usually have lower boiling and melting points. An amorphous solid doesn't have a crystalline structure and has an irregular bonding pattern. Although glass seems like a crystalline solid, it softens and melts at a range of temperatures. Amorphous solids include glass, plastic and tar.