MOLECULAR SHAPE AND STRUCTURE

3 (a) The shape is angular the electron pair on the central Cl atom results in a 

5 (a) The shape of the thionyl chloride molecule is trigonal pyramidal, (b) The 

O—S—Cl angles are identical. The lone electron pair repels the bonded electron pairs equally and thus, all O—S—Cl bond angles are compressed equally, (c) The expected bond angle is slightly less than 109.5°. 

23 (a) Of the three forms, 1 and 2 are polar only 3 is nonpolar. This is because the 

C—Cl bond dipoles are pointing in exactly opposite directions in 3. (b) The dipole moment for 1 would be the largest because the C—Cl bond vectors are pointing most nearly in the same direction in 1 (60° apart) whereas in 2 the C—Cl vectors point more away from each other (120°), giving a larger cancellation of dipole. 

1 The Basic VSEPR (Valence-Shell Electron-Pair Repulsion) Model 

Typical Electron Arrangements in Molecules with One Central Atom 

X = n atoms same or different) bonded to central atom E , = lone pairs on central atom Useful for generalizing types of structures 

Note Molecular shape is defined by the location of the atoms alone. The bond angles are Z X-A-X. 

Count the number of electron pairs (bonding and nonbonding) around the central atom(s). 

Regions of high electron density that repel each other (Coulomb s law) by rotating about a central atom, thereby maximizing their separation 

Angle(s) between bonds joining atom centers 

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When a polymer is extruded through an orifice such as a capillary die, a phenomenon called die swell is often observed. In this case, as the polymer exits the cylindrical die, the diameter of the extrudate increases to a diameter larger than the diameter of the capillary die, as shown in Fig. 3.9. That is, it increases in diameter as a function of the time after the polymer exits the die. Newtonian materials or pure power law materials would not exhibit this strong of a time-dependent response. Instead they may exhibit an instantaneous small increase in diameter, but no substantial time-dependent effect will be observed. The time-dependent die swell is an example of the polymer s viscoelastic response. From a simplified viewpoint the undisturbed polymer molecules are forced to change shape as they move from the large area of the upstream piston cylinder into the capillary. For short times in the capillary, the molecules remember their previous molecular shape and structure and try to return to that structure after they exit the die. If the time is substantially longer than the relaxation time of the polymer, then the molecules assume a new configuration in the capillary and there will be less die swell. 

It is assumed that the reader has previously learned, in undergraduate inorganic or physical chemistry classes, how symmetry arises in molecular shapes and structures and what symmetry elements are (e.g., planes, axes of rotation, centers of inversion, etc.). For the reader who feels, after reading this appendix, that additional background is needed, the texts by Cotton and EWK, as well as most physical chemistry texts can be consulted. We review and teach here only that material that is of direct application to symmetry analysis of molecular orbitals and vibrations and rotations of molecules. We use a specific example, the ammonia molecule, to introduce and illustrate the important aspects of point group symmetry. 

In chemical reactions electrons move from full to empty orbitals Molecular shape and structure determine reactivity Representing the movement of electrons in reactions by curly arrows... 

Molecular shape and structure Enols and enolates ch21, ch25-ch29... 

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