Covalent Lewis Structures. Add up the valence electrons. Put the element with the fewest number of atoms in the middle and surround with the remaining elements. Draw a line from the central atom to the adjacent atoms Subtract two electrons for each line drawn.
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Formal Charge = # of valence electrons – (# of dots+ # of lines)
Steps for Drawing Lewis StructuresAdd up the total number of valence electrons available (EA). Be sure to take into account the total number of atoms present. Exception: if the compound is negatively charged, add 1 electron for each negative valueException: if the compound is positively charged, remove 1 electron for each negative valueExample: CH4 C= 4 VE H= 1VE = 4 + 4(1) = 8 EAExample: CO2Example: SO42-Draw out a skeletal arrangement of lewis structure. Place the least electronegative atom in the center, surrounded by the remaining atoms. Draw lines to represent the bonds between the central atom and each surrounding atom. Exception: Hydrogen can never be a central atom because it is found in the first energy level and can only have 2 electrons at most. Draw the skeletal arrangement in the following examplesCO2 H2SO4 (Just underline the central atom)NCl3 PF5 Count and draw in the number of electrons necessary (EN) for each to have a full octet. These are the electrons needed. Be sure to include the bonded electrons. Example: CO2Subtract the number of valence electrons available (from step one) from the number of electrons needed (EN –EA). This is the number of valence electrons that you still need. We make up for these by adding double or triple bonds. If you get a negative number, you have extra electrons. These may become part of your expanded octet.Example: CO2To add double or triple bonds: divide the number you get from step 4 by 2 (why? Because we are adding more bonds and each bond represents 2 electrons). The number you get is the number of bonds you need to add in the form of either double or triple bonds. Example: CO2