Teaching Chemistry with Models

By R. T. Sanderson | Go to book overview
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CHAPTER 4
GENERAL APPLICATIONS OF MOLECULAR, IONIC, AND CRYSTAL MODELS

MOLECULAR GEOMETRY

The directional nature of covalence has already been discussed in terms of their representation by atomic models. With molecular models in addition, the picture can be completed.

For example, atoms capable of forming only two covalent bonds are chiefly of two types, exemplified by beryllium and oxygen. A model of a gas molecule of BeCl2 or BeF2 (Fig. C-11) or any similar compound will show the linear nature of the bonding that results when all outer electrons are used in forming two bonds. Models of CO2, acetylene, and similar compounds are useful in showing how this principle includes multiple bonding. The effect of two unshared electron pairs is shown well by models of H2O, H2S, SCl2, Cl2O, OF2, (CH3)2S, and many other compounds. Extension to multiple bonding is demonstrated by models of ozone, SO2, SeO2, and others; in SO2 there is only one unshared electron pair, so the bond angle is 120° (Fig. C-7).

When all outermost electrons are involved in only three bonds, the planar triangular structure predictable from the atomic models of boron and aluminum and gallium results. Models of BF3, BCl3 (Fig. C-11), and B(CH3)3 are helpful in illustrating this. Inclusion of multiple bonding adds models of nitric acid, NO3-, ethylene or other olefins, formate or any other carboxylate ion, SO3, benzene, borazene, carbonate ion, COCl2, and many others.

To demonstrate the pyramidal structure predictable from atomic models of VA elements, where an unshared electron pair must be taken into account, models of NH3, NCl3, NF3, PCl3, PBr3, PH3, PF3 (Fig. C-11), and others, are suggested. Inclusion of multiple bonding adds to this list models of such compounds or ions as ClO3-, SO3=, and SOCl2.

The tetrahedral structure indicated by atomic models of carbon and silicon is shown in models (Fig. C-14) of CH4, SiH4, CH3OH, CCl4, CF4, SiF4, SiCl4, diamond, CH3F, SiH3Cl, and any of hundreds more. The

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