Chemical Bonding Worksheets Page 11
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22Lewis (Electron Dot) Diagrams for POLYATOMIC IONS—GUIDELINES
1. Calculate the total number of valence electrons for all the atoms.
2. Account for # of electrons associated with charge:
– If ion is positively charged, subtract # of electrons from total
→
– If ion has +2 charge
subtract 2 electrons from total to get the total # of electrons
– If ion is negatively charged, add # of electrons from total
→
– If ion has –3 charge
add 3 electrons to get the total # of electrons
3. Divide new total by 2 to get total # of electron pairs
4. Surround central atom (will be indicated) with 4 electrons pairs, then distribute outer atoms
around central atom.
5. If any atom (except H) does not have an octet, move nonbonding electrons from the central
atom to a position b/w atoms, forming double and triple bonds until all atoms have an octet.
6. Put brackets around all the atoms, and put charge on upper right-hand side
– This indicates the charge belongs to entire entity rather than to a single atom in the ion.
Example: Draw the Lewis Structure for each of the following polyatomic ions:
+
–
a. NH
c. BrO
4
3
b. SO
–2
d. PO
–3
4
4
CHEM 121 Tro Chapter 10 F2012
page 11 of 22
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