CHEM 121L General Chemistry Laboratory Revision 1 5 Molecular Modeling of Covalent Compounds To learn about the geometry of covalently bound molecules To learn about VSEPR theory To learn about Isomerism To learn about Molecular Polarity In this laboratory exercise we will build models of some simple molecules that are in accordance with the geometries suggested by the Valence Shell Electron Pair Repulsion VSEPR Theory We will note the influence Lone Pairs of electrons and Multiple Bonds have upon the geometry of these molecules We will also note the overall polarity of the molecules modeled VSEPR theory provides a simple extension of Lewis bonding theory predicting molecular geometries for covalently bound molecules polyatomic ions and networks This theory assumes the shape of a molecule is influenced by the number of electron pairs about each central atom in the molecule Although largely accurate in predicting molecular geometries it is a bit superficial and must be supplemented with Valence Bond Theory in order to provide a more accurate picture of the orbital structure of said electron pairs In any case both VSEPR and Valence Bond Theory are being supplanted by the more robust although more computationally demanding Molecular Orbital Theory to describe both the electronic structure and geometry of simple molecules The basic idea underlying VESPR is that each valence shell electron pair around an atom will mutually repel all the other valence shell electron pairs about that atom Therefore the electron pairs and covalent bonds will find a geometric arrangement which minimizes these repulsions These arrangements for the common cases are Num Electron Domains 2 3 4 5 6 Geometry Angle Between Domains Linear 180o Trigonal Planar 120o Tetrahedral 109 5o Trigonal Bipyramidal 120o equatorial 90o axial Octahedral 90o This Electronic Geometry defines the type of Molecular Geometry possible For each type of Electronic Geometry there is a subset of Molecular Geometries which depend on the number of atoms covalently bound to the central atom as well as the number of Lone Pairs about that atom As an example each of the following molecules CH4 NH3 H2O and HF is tetrahedral in its Electronic Geometry each has four electron domains about the central atom However they each exhibit different Molecular Geometries CH4 is tetrahedral but NH3 is trigonal pyramidal Page 2 CH4 has 4 bonding atoms about the C atom whereas NH3 has only 3 with the fourth electron pair being a Lone Pair When determining Molecular Geometries Lone Pairs of electrons are not considered The Lone Pairs influence the Molecular Geometry but do not participate in it A list of possible Molecular Geometries for 2 6 electron pairs about a central atom E is provided below Page 3 Several points need to be considered when identifying Molecular Geometries i Double and triple bonds count as a single VSEPR domain This is because a second covalent bond between two atoms shortens and strengthens the overall bonding but it does not change the geometry of the bonding For the series CH3CH3 CH2CH2 CHCH the Carbon Carbon bond length gets progressively shorter simply bringing the Carbon atoms closer together In the example below the central Carbon atom has three VSEPR domains and so is trigonal planar in its geometry Page 4 ii Lone Pair Lone Pair interactions are more unfavorable than Lone Pair Bonding Pair interactions Likewise these last are more unfavorable than Bonding Pair Bonding Pair interactions So Xenon Tetrafluoride XeF4 which has 4 Bonding Pairs and 2 Lone Pairs about the central Xenon atom adopts a Square Planar molecular geometry iii Molecules with multiple central atoms will have multiple geometries So CH3SH has two central atoms the carbon and sulfur Thus it has two specified geometries It is tetrahedral about the carbon 4 Bonding Pairs and bent about the sulfur 2 Bonding Pairs and 2 Lone Pairs iv The bond angles in a molecule are determined primarily by the Electronic Geometry So both CH4 and NH3 have approximate bond angles of 109 5o It is true the Lone Pair in NH3 requires more space than the Bonding Pairs the measured bond angle in NH3 is 107o However as a first approximation we can take the bond angles to be those specified by the electron domains listed above Page 5 Thus the process for determining the Molecular Geometry of a covalently bound compound is Write the Lewis Structure for the molecule Determine the Electronic Geometry of the molecule Determine the Molecular Geometry An example is in order Consider the compound Nitrogen Trichloride NCl3 A valid Lewis Structure for this compound is The central Nitrogen atom has four electron domains three Bonding Pairs and one Lone Pair This gives rise to a Tetrahedral Electronic Geometry Four electron domains with one Lone Pair as can be seen in the Table above gives rise to a Trigonal Pyramidal Molecular Geometry Finally the molecular geometry plays an important role in determining if the molecule is Polar For instance both BF3 and GeF2 have very polar bonds This is because Fluorine is very Electronegative and the metalloids B and Ge are somewhat Electropositive However BF3 because of its symmetric Trigonal Planar structure is non polar whereas GeF2 adopting an asymmetric Bent structure is polar Page 6 In this laboratory exercise we will build models of molecules for several compounds This will help us visualize the three dimensional structure of these molecules It should be emphasized chemical reactions occur in three dimensions and so inherently depend on the three dimensional structure of the reactants This is especially important in biochemistry where enzymes are specifically designed for chemical substrates with a particular three dimensional structure Page 7 Pre Lab Questions 1 Draw Lewis Structures for each of the molecules and ions in the Procedure section of the lab You may skip the Lewis Structure for Morphine If you do this now your time spent in the lab will be much shorter Page 8 Procedure We will use two different model kits for the following exercises The Prentice Hall Molecular Model Set for General and Organic Chemistry will be used for simpler cases involving atoms that adopt Tetrahedral Trigonal Planar and Linear Electronic Geometries It will also be used for molecules that contain multiple geometric centers For molecules containing Trigonal Bipyramidal and Octahedral centers the Indigo Instruments Model Kit will be used For the following cases check out and