One way in which the shapes of molecules manifest themselves experimentally is through molecular dipole moments. You may examine several Jmol models of compounds discussed above by. A pop-up menu of commands may be accessed by the right button on a PC or a control-click on a Mac while the cursor is inside the display frame. To measure a torsion angle, do a double-click, single-click, single-click, double-click on four atoms. To measure a bond angle, do a double-click, single-click, double-click on three atoms. To measure a distance, double-click on two atoms. Atom distances and angles are easily determined. This powerful visualization tool allows the user to move a molecular stucture in any way desired. Some of the useful features of physical models can be approximated by the model viewing applet Jmol. Many kinds of model kits are available to students and professional chemists. The best way to study the three-dimensional shapes of molecules is by using molecular models. Click on the university name to visit their site. Nice treatments of VSEPR theory have been provided by Oxford and Purdue. The compound boron trifluoride, BF 3, does not have non-bonding valence electrons and the configuration of its atoms is trigonal. Of course, it is the configuration of atoms (not electrons) that defines the the shape of a molecule, and in this sense ammonia is said to be pyramidal (not tetrahedral). The measured bond angles of these compounds (H 2O 104.5ยบ
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