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  The Shape of Molecules Homework Help - K-12 Grade Level, College Level Chemistry

Introduction of the Shape of Molecules

A molecule's three dimensional shape or configuration is a significant characteristic. On the preferred spatial orientation of covalent bonds to atoms having two or more bonding partners, this shape is dependent. The 3-d configurations are best shown with the aid of models. Consecutively, to represent this kind of configurations on a 2-d surface (paper, blackboard or screen), we use perspective drawings. In perspective drawings the direction of a bond is specified by the line connecting the bonded atoms. 

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In many cases the configuration's focus is a carbon atom so the lines specifying bond directions will originate there. A bond lying represents by the simple straight line, approximately in the surface plane. Two bonds to substituents A in the structure on the left are of this type. The bond that is Wedge shaped is directed in the front of this plane (thick end toward the viewer), as displayed by bond to substituent B and in back of the plane that is away from the viewer, a hatched bond is directed, as shown by bond to substituent D. Some text and other sources can use a dashed bond in similar manner as we have described the hatched bond but this can be confusing because dashed bond is frequently used to represent a partial bond (that is a covalent bond that is partially formed or partially broken). Following cases make use of this notation and also defines the importance of including non-bonding valence shell electron pairs (colored blue) when viewing such type of configurations.

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By valence-shell electron-pair repulsion theory Bonding configurations are readily predicted, usually referred to as VSEPR in most introductory chemistry texts. This simple model is based on fact that electrons repel each other and that it is reasonable to expect that the bonds and non-bonding valence electron pairs that are associated with a given atom will prefer to be as far apart as possible. Carbon's Bonding configurations are easy to remember, because there are only 3 categories that are shown in the following table:

Configuration

Bonding Partners

Bond Angles

Example

Tetrahedral

4

109.5º

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Trigonal

3

120º

Linear

2

180º

In the three examples shown in the diagram, the central atom that is carbon does not contain any non-bonding valence electrons; therefore the configuration may be approximated from the number of bonding partners alone. For the water and ammonia's molecules, however, non-bonding electrons must include in the calculation. In each case there are four regions of electron density that are associated with the valence shell so that a tetrahedral bond angle is expected. Measured bond angles of these compounds (H2O 104.5º & NH3 107.3º) demonstrates that they are closer to being tetrahedral than linear or trigonal. Obviously, it is the atoms' (not electrons) configuration that describes the shape of a molecule and in this manner the ammonia is said to be pyramidal (not tetrahedral). The configuration of Compound boron trifluoride BF3 atoms is trigonal and does not have the non-bonding valence electrons and. 

 By using molecular models we can study the 3-d shapes of molecules, and this is the best way of study. Many types of the model kits are presents for students and professional chemists. By the model viewing applet Jmol some of useful features of physical models can be approximated. Jmol is most powerful visualization tool that permits the user to move a molecular stucture in any way they desired. Atom distances and angles are without any difficulty can be determined. By double-click on the two atoms we can measure a distance. To measure a bond angle, do double-click, single-click, double-click on the three atoms. By doing double-click, a single-click, a single-click, double-click on the four atoms we can measure a torsion angle. Pop-up menu of commands may be accessed through the right button on a PC or a control-click on a Mac while the cursor is inside the display frame.

One way in which the shapes of molecules manifest themselves experimentally is via the molecular dipole moments. Molecule that has one or more polar covalent bonds can consist a dipole moment as a result of the accumulated bond dipoles. In example of water, the O-H covalent bond is polar, due to the dissimilarities between the electronegativities of hydrogen and oxygen. Because there are two O-H bonds in the water, their bond dipoles will interact and may result in a molecular dipole which can be measured. The picture displays four possible orientations of the O-H bonds.

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Bond dipoles are colored magenta and in the blue color there is resulting molecular dipole. The bond dipoles cancel and the molecular dipole is zero in linear configuration (bond angle 180º). For other bond angles (120 to 90º) molecular dipole would be not same in size, being largest for the 90º configuration. In a same manner the configurations of methane (CH4) and carbon dioxide (CO2) may be deduced from their zero molecular dipole moments. Because the bond dipoles have cancelled, the configurations of these molecules have to be linear and tetrahedral (or square-planar) respectively.
Example of methane provides insight to other arguments that have been used to confirm its tetrahedral configuration. For purposes of the discussion we shall consider the three other configurations for the CH4, square-planar, square-pyramidal and triangular-pyramidal. 

As the replacement of the one hydrogen by chlorine atom gives a CH3Cl compound. Because square-planar, tetrahedral and square-pyramidal configurations have structurally equal hydrogen atoms, they would each one gives single substitution product. Though, in the trigonal-pyramidal configuration one hydrogen (the apex) is structurally not same from the other three (the pyramid base). The Substitution in this example should give two distinct CH3Cl compounds if all the hydrogens react. In case of disubstitution, the tetrahedral configuration of methane would lead to a single CH2Cl2 product, but other configurations would give two different CH2Cl2 compounds.

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