1. 8.5 Lewis Structures By Ali & Sam

2. Drawing Lewis Structures (Review) 1) Add up all the valence electrons 1) Add up all the valence electrons If it is an anion (-), add an electron If it is an anion (-), add an electron If it is a cation (+), subtract an electron If it is a cation (+), subtract an electron 2) Write the symbols for the atoms and connect them together with bonds (dashed lines) 2) Write the symbols for the atoms and connect them together with bonds (dashed lines) 3) Complete the octets 3) Complete the octets Octet is complete with 8 electrons Octet is complete with 8 electrons Hydrogen is complete with 2 Hydrogen is complete with 2 4) Place the leftover electrons on central atom 4) Place the leftover electrons on central atom 5) If there are not enough electrons try multiple bonds 5) If there are not enough electrons try multiple bonds

3. Formal Charge (New information) Formal Charge- “The formal charge of an atom is the charge that an atom in a molecule would have if all atoms had the same electronegativity (that is, if all bonding electron pairs were shared equally between atoms). (pg 293) Formal Charge- “The formal charge of an atom is the charge that an atom in a molecule would have if all atoms had the same electronegativity (that is, if all bonding electron pairs were shared equally between atoms). (pg 293)

4. How to Calculate Formal Charge 1) All unshared electrons go to the atom on which they were found 1) All unshared electrons go to the atom on which they were found 2) half of the bonded electrons go to each atom in the bond 2) half of the bonded electrons go to each atom in the bond “The formal charge of an atom equals the number of valence electrons in the isolated atom, minus the number of electrons assigned to the atom in the Lewis Structure.” (pg 293) “The formal charge of an atom equals the number of valence electrons in the isolated atom, minus the number of electrons assigned to the atom in the Lewis Structure.” (pg 293) # of valence electrons-# of electrons in Lewis Dot Diagram # of valence electrons-# of electrons in Lewis Dot Diagram The most stable Lewis Structures are the ones with formal charges closest to zero and has the negative charges on the electronnegative atoms. The most stable Lewis Structures are the ones with formal charges closest to zero and has the negative charges on the electronnegative atoms.

5. Example (pg 293) Ex) Cyanide ion => CN ֿ Ex) Cyanide ion => CN ֿ Lewis Structure => [:C≡N:] ֿ C has 4 valence electrons. It has 2 non- bonding electrons and 3 bonded electrons that equal 5, so 4-5= -1. Therefore the formal charge for C is -1. N has 5 valence electrons. It has 2 non-bonding electrons and 3 bonded electrons that equal 5. 5-5=0, so the formal charge of N is 0. The sum of the overall charge of the ion CN ֿ has a formal charge of -1.

6. What Lewis Structures are preferred Three possible Lewis structures of the thiocyanate ion, NCSֿ, are; Three possible Lewis structures of the thiocyanate ion, NCSֿ, are; [:N-C ≡S:] ֿ [N=C=S] ֿ [:N ≡C-S:] [:N-C ≡S:] ֿ [N=C=S] ֿ [:N ≡C-S:] -2 0 +1 -1 0 0 0 0 -1 -2 0 +1 -1 0 0 0 0 -1 To determine which one is preferred we need to figure out the formal charges. N is more electronegative, so the middle one is more preferred. The 2 nd and 3 rd one have formal charges closest to zero, but the negative charge has to be on the most electronegative element. (sample exercise 8.9)

7. Bibliography Brown and LeMay Chemistry: The Central Science. New Jersey: Pearson Education, Inc. 2003 Brown and LeMay Chemistry: The Central Science. New Jersey: Pearson Education, Inc. 2003