11.7 Solids There are two groups of solids: Crystalline solids The atoms, ions, or molecules are ordered in well-defined arrangements. These solids have flat surfaces, or faces —particles are in highly order, that make definite angles with each other. These solids have highly regular shapes (see Fig. 11.29).

Amorphous solids A solid whose particles have no orderly structure. They lack well-defined faces and shapes. Examples include glasses, rubber, etc.

11.8 Types of Bonding in Crystalline Solids

Covalent-network solids consist of atoms held together by covalent bonds. Diamonds are an example of a covalent-network solid in which atoms are covalently bonded to each other. They tend to be hard and have high melting points.

Molecular solids consist of atoms or molecules held together by intermolecular forces. Graphite is an example of a molecular solid in which layers of atoms are held together with van der Waals forces. Also: sucrose, dry ice, water, for example They tend to be softer and have lower melting points.

Ionic solids Consist of ions held together by ionic bonds. The strength of the ionic bond (see discussion of lattice energy in sec 8.2) depends greatly on the charges of the ions. NaCl, with charges of 1+ and 1-, melts at 801oC, whereas MgO, with charges of 2+ and 2-, melts at 2852oC. Ionic compounds have an orderly arrangement of ions. Their structures depends on the charges and relative sizes of their ions.

Metallic Solids Consist entirely of metal atoms. Metals are not covalently bonded, but the attractions between atoms are too strong to be van der Waals forces. In metals, valence electrons are delocalized throughout the solid.