Electronegativity three-dimensional representation

Important topics in Chapter 1 include drawing Lewis structures, predicting the shap)e of molecules, determining what orbitals are used to form bonds, and how electronegativity affects bond polarity. Equally important is Section 1.7 on drawing organic molecules, both shorthand methods routinely used for simple and complex compounds, as well as three-dimensional representations that allow us to more clearly visualize them. 

Now draw the Lewis diagram for NH3, with the arrows pointing to the more electronegative element. Also sketch the three-dimensional representation of the trigonal pyramidal structure of the molecule. Is NH3 polar or nonpolar ... 

The bonding electrons in ammonia are displaced toward the more electronegative nitrogen atom. The bonds do not cancel in the asymmetrical pyramidal shape, so the molecule is polar. The three-dimensional representation, which attempts to show the molecular shape, better suggests the charge displacement toward the nitrogen atom. 

For the simple binaiy compounds discussed in Sections II.A. I and II.A.2, at given P and T, usually the partial pressure of the electronegative element is used. In temaiy compounds a second component activity has to be determined, leading to a three-dimensional representation for the relationships between defect concentrations and component activities. 

Non-pharmaceutical applications tend to be aimed at accurate stmcture prediction together with properties such as electrostatic potential maps and surface hydrophobic-ity. One consequence of this is a desire for easily understood graphical representations of both the molecules and the calculated properties. An example of this can be seen in Figure 1, which shows a geometry-optimized computed structure of the potassium complex of the macrocycle 18-crown-6. A representation of the complex s electrostatic potential on a van der Waals surface is overlaid on top of this. A cutaway view is depicted so that the connectivity and identity of each atom can be seen clearly and correlated with the local electrostatics. In the example, blue represents an electropositive region, red is electronegative, and green is neutral. As the complex can be rotated on a computer screen, the structure can be considered to be pseudo three dimensional. The representation of physical properties adds a further dimension to this. Snapshots of molecules and associated properties, perhaps modeled at precise intervals... 

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