Name Atomic Theory Unit Drawing Lewis Structures I Chemists follow several guidelines when visually representing covalent compounds The conventional method of drawing produces Lewis structures Work together to draw the compounds using these Lewis structure steps Determine total number of valence electrons Predict total number of Bonds S N A rule Draw Skeletal Structure Place nonbonding electrons Fix the number of bonds 1 C2H6 2 C2H4 3 C2H2 Needed 2C 2 8 16 6H 6 2 12 Total Needed 28 Needed 2C 2 8 16 4H 4 2 8 Total Needed 24 Needed 2C 2 8 16 2H 2 2 4 Total Needed 20 Available 2C 2 4 8 6H 6 1 6 Total Available 14 Available 2C 2 4 8 4H 4 1 4 Total Available 12 Available 2C 2 4 8 2H 2 1 2 Total Available 10 Number of bonds Shared N A Shared 28 14 14 bonds shared 2 bonds 7 bonds Number of bonds Shared N A Shared 24 12 12 bonds shared 2 bonds 6 Number of bonds Shared N A Shared 20 10 10 bonds shared 2 bonds 5 Revised SH 7 7 13 LaBrake Vanden Bout 2013 Name II Often there are multiple structures possible to represent the same set of atoms The molecular formula can sometimes give insight on how to connect the atoms in the structure Draw the following Lewis structures Compare with your neighbors Specifically compare drawings for number 4 with numbers 5 and 6 4 C2H6O 5 CH3OCH3 6 C2H5OH Drawings for 4 could look like either of the structures drawn in 5 or 6 For each structure the number of needed and available electrons is the same Needed 2C 2 8 16 6H 6 2 12 1O 1 8 8 Total Needed 36 Available 2C 2 4 8 6H 6 1 6 1O 1 6 6 Total Available 20 Number of bonds Shared N A Shared 36 20 16 bonds shared 2 bonds 8 bonds Revised SH 7 7 13 LaBrake Vanden Bout 2013 Name III Draw two different structures for the following compound How can you be sure which structure gives the best representation of the compound Make your best guess as to which structure is better and we will discuss the concept of formal charge together 7 COCl2 8 COCl2 again Both of these structures have a total formal charge of 0 However we look for the structure with the lowest formal charge on all atoms So we would select the drawing under 8 as the correct structure for this molecule For each structure the number of needed and available electrons is the same Needed 1C 1 8 8 2Cl 2 8 16 1O 1 8 8 Total Needed 32 Available 1C 1 4 4 2Cl 2 7 14 1O 1 6 6 Total Available 24 Number of bonds Shared N A Shared 32 24 8 bonds shared 2 bonds 4 bonds Revised SH 7 7 13 LaBrake Vanden Bout 2013 Name IV Compounds that exhibit resonance have structures that are the averaged combination of multiple drawn structures Draw the different resonance structures for this compound 9 NO3 Averaged Structure For structures with a charge like 1 in this case you add or subtract extra electrons to or from the available pool For negative charges we add electrons For positive charges we subtract electrons Needed 1N 1 8 8 3O 3 8 24 Total Needed 32 Available 1N 1 5 5 3O 3 6 18 1 charge 1 1 1 Total Available 24 Number of bonds Shared N A Shared 32 24 8 bonds shared 2 bonds 4 bonds Revised SH 7 7 13 LaBrake Vanden Bout 2013 Name V Finally the Lewis structures rules do not always work Attempt the next compounds and give a reason for why the steps broke down for each example 10 NaOH 11 RnCl2 12 BeCl2 Reason Reason Reason Although the Lewis structure guidelines would be helpful for the OH group NaOH is Ionic and we would represent it as a lattice structure rather than a line drawing Na does not share any electrons with the OH group and vice versa Expanded Valence Too many available electrons Needed Needed 2Cl 2 8 16 1Rn 1 8 8 Total Needed 24 2Cl 2 8 16 Available 2Cl 2 7 14 1Rn 1 8 8 Total Available 22 Available 2Cl 2 7 14 1Be 1 2 2 Total Available 16 Number of bonds Shared N A Shared 24 22 2 bonds shared 2 bonds 1 bond We know there must be at least TWO bonds to connect 3 atoms Then we distribute the remaining available electrons Revised SH 7 7 13 Incomplete Valence Too few available electrons Beryllium only needs 4 electrons 1Be 1 4 4 Total Needed 20 Number of bonds Shared N A Shared 20 16 4 bonds shared 2 bonds 2 LaBrake Vanden Bout 2013
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