Cations covalent network

CQ, ) wjt an(j q COvalently bonded together the complex ion also interacts ionically with Ca2+. Such complex ions need not be discrete entities but can form polymeric covalent networks with a net charge, with ionic bonds to cations (e.g. silicates see Topics D6 and F4). Even when only two elements are present, however, bonding may be hard to describe in simple terms. 

The substitution of aluminum for silicon in a silica covalent network leads to a charge unbalance, which must be compensated by extra-framework cations, mostly alkaline. This occurs in the cases of the so-called stuffed silicas these materials have structures strictly related to the crystalline forms of silica, but with cations in the interstices to counterbalance the presence of A1 ions substituting for Si. This is the case, for example, of Eucriptite (LaAlSi04, a stuffed /3-quartz) or nepheline (NaAlSiOa, a stuffed tridymite). A similar mechanism also occurs in the amorphous networks of glasses [175]. 

Interaction with plurivalent cations via ligand exchange mechanism is one more rather widely applied crosslinking technique. The network bonds of ionic or donor-acceptor nature are located, with respect to lifetime, between the truly covalent crosslinks and physical entanglements. Generally speaking, gelation in these systems is reversible. 

The concept of silicates as inorganic polymers was implicit in the ideas developed by W. H. Zacheriasen in the early 1930s. He conceived of silicates as consisting of macromolecular structures held together by covalent bonds but including network-dwelling cations. These cations were not assumed to have a structural role but merely to be present in order to balance the charges on the anionic polymer network. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

It is useful as a point of departure, to briefly describe the basic crystal lattice common to phyllosilicates. The elementary character is the SiO tetrahedral linkage of an essentially two-dimensional, hexagonally symmetric, network. One side of this "sheet network is coordinated with other cation-oxygen complexes joined by an important component of covalent bonding while the other is coordinated by essentially ionic bonding or van der Waals type bonds. The key to phyllosilicate structures is the oxygen network which determines the shape and extent of the structure. 

Ans. (a)Ba—denser electron sea (b) Si—covalently bonded network versus molecular crystal (c)Xe—higher atomic number means stronger London forces (d) MgF2—cations and anions both smaller. 

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