Atomic bonds Clays

In Chapter 4 we learned how silicates give clay its plastic properties. In this chapter we will see how carbon atoms bond in chain arrangements to produce organic compounds that can have the plastic properties of clay. We will study sculpture and describe molecular forms as sculptures. Finally, we will see how organic molecules, microscopic sculptures, have the same properties as macroscopic sculptures, which are visible to the naked eye. 

Two types of swelling may occur. Surface hydration is one type of swelling in which water molecules are adsorbed on crystal surfaces. Hydrogen bonding holds a layer of water molecules to the oxygen atoms exposed on the crystal surfaces. Subsequent layers of water molecules align to form a quasi-crystalline structure between unit layers which results in an increased c-spacing. All types of clays swell in this manner. 

Figure 3.2. A silicon tetrahedron (left), an aluminum octahedron (middle) as a central layer in a 2 1 clay, and an aluminum octahedron (right) as a surface layer in a 1 1 clay (right). The oxygen atoms are bonded to other silicon and aluminum atoms in the clay (bonds are not intended to be shown at the correct angles). Below is a water molecule showing partially positive hydrogen atoms and partially negative oxygen atoms. Also shown are the two lone pairs of electrons on all the oxygen atoms.

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

A variety of mineral-type materials are inorganic polymers [Ray, 1978]. Silica [(SiCLL] is found in nature in various crystalline forms, including sand, quartz, and agate. The various crystalline forms of silica consist of three-dimensional, highly crosslinked polymer chains composed of Si04 tetrahedra where each oxygen atom is bonded to two silicon atoms and each silicon atoms is bonded to four oxygen atoms. Silicates, found in most clay, rocks, and... 

Make a model of the bonding structure using clay and toothpicks. The clay should represent atoms or ions and the toothpicks should represent bonds. Your model should have at least one structural unit. The additive method of three-dimensional construction should be used. 

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