Molecular shapes prediction

EXAMPLE 3.3 Sample exercise Predicting a molecular shape Predict the shape of a sulfur tetrafluoride molecule, SF4. 

Fig. 17.3 Molecular shapes predicted by simple VSEPR theory Bond angle values represent experimental results where known...

Molecular shapes predicted by VSEPR theory include linear, bent, trigonal planar, tetrahedral, and trigonal pyramidal. 

TABLE 3.8 Molecular Shapes Predicted by the Valence Shell Electron-Pair Repulsion Theory... 

Describe the five electron-group arrangements and associated molecular shapes, predict molecular shapes from Lewis structures, and explain deviations from ideal bond angles ( 10.2) (SPs 10.6-10.8) (EPs 10.25-10.49)... 

For reasons outlined earlier, we expect even less accuracy in EH calculations of molecular deformations involving bond-angle or bond-length changes. This expectation is generally reinforced by calculation. Molecular shape predictions become poorer... 

On the assumption that the pairs of electrons in the valency shell of a bonded atom in a molecule are arranged in a definite way which depends on the number of electron pairs (coordination number), the geometrical arrangement or shape of molecules may be predicted. A multiple bond is regarded as equivalent to a single bond as far as molecular shape is concerned. 

Predict the formula and molecular shape of a hydride of phosphorus. 

Recently, a symmetry rule for predicting stable molecular shapes has been developed by Pearson Salem and Bartell" . This rule is based on the second-order, or pseudo, Jahn-Teller effect and follows from the earlier work by Bader . According to the symmetry rule, the symmetries of the ground state and the lowest excited state determine which kind of nuclear motion occurs most easily in the ground state of a molecule. Pearson has shown that this approximation is justified in a large variety of inorganic and small organic molecules. 

Predicting and sketching molecular shapes Predi cting and explaining bond angles Identifying molecular polarity... 

First we draw the Lewis structure of each species, then use these Lewis structures to predict the molecular shape. 

In summary, the Lewis-like model seems to predict the composition, qualitative molecular shape, and general forms of hybrids and bond functions accurately for a wide variety of main-group derivatives of transition metals. The sd-hybridization and duodectet-rule concepts for d-block elements therefore appear to offer an extended zeroth-order Lewis-like model of covalent bonding that spans main-group and transition-metal chemistry in a satisfactorily unified manner. 

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20-50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. 

Thus, dendrimers exhibit a unique combination of (a) high molecular weights, typical for classical macromolecular substances, (b) molecular shapes, similar to idealized spherical particles and (c) nanoscopic sizes that are larger than those of low molecular weight compounds but smaller than those of typical macromolecules. As such, they provide unique rheological systems that are between typical chain-type polymers and suspensions of spherical particles. Notably, such systems have not been available for rheological study before, nor are there yet analytical theories of dense fluids of spherical particles that are successful in predicting useful numerical results. 

Use your knowledge of molecular shape and polar bonds to predict whether each molecule has an overall molecular polarity. 

You can use soap bubbles to simulate the molecular shapes that are predicted by VSEPR theory. The soap bubbles represent the electron clouds surrounding the central atom in a molecule. 

Follow the four-step procedure that helps to predict molecular shape. Use Table 4.2 for names of the electron-group arrangements and molecular shapes. 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

Use VSEPR theory to predict the molecular shape for each of the following ... 

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