These are organized in particular groupings to form molecules, crystals or particles acting as micro-units from which the physical structure of any solid material is constructed. All physical and chemical changes in a material originate on the scale of the atom or micro-unit, whereas they are normally observed on the scale of man's visual perception. When any event in a material is observable, it has progressed to a stage where millions of atoms are involved. For this reason it is useful to establish a relative scale for comparison. Various assemblages of atoms that form the structural micro-units of materials are usually less than a micron in size and thus must be magnified more than 100 times to be seen.
Atoms are the basic chemical building units.
For the purpose of this Digest it is sufficient to note that the chemical nature of a material is determined by the basic chemical unit, the molecule. The particular elements and their arrangement as well as the number of atoms determine the chemical nature of the molecule. Some general aspects of organic materials are discussed in CBD 117. In the major inorganic class of materials such as clays and cement the common backbone of the silicates is the silica tetrahedron. This unit forms a tetrahedron shape and permits a great variety of structural configurations by joining at the apex of adjacent units.