Miss Mayer
Chemistry 30
Resonance Structures
Sometimes there is more than one way to draw a Lewis structure following all of the rules.
Example:
Ozone
In the diagram we drew, ozone appears to have a single bond and one double bond.
We would expect that is one bond was a double bond, then it should be SHORTER that the
other (single) bond because the “pull” is greater.
However, it has been discovered that the bond lengths between the central oxygen and the
other two oxygens are actually IDENTICAL.
Since all of the atoms are identical (oxygen) which atom is chosen for the double bond?
These Lewis structures are equivalent except for the placement of the electrons (ex. the
location of the double bond).
Equivalent Lewis structures are called ____________________________________________
The correct way to describe ozone as a Lewis structure would be:
This indicates that the ozone molecule is described by _________________ of the two Lewis
structures (the resonance forms)
Miss Mayer
Chemistry 30
The important points to remember about resonance forms are:
- the molecule is not rapidly oscillating (flipping) between the different forms
- there is only one form of the ozone molecule, and the bond lengths between the oxygens
are intermediate between characteristic single and double bond lengths between a pair of
oxygens
- we draw two Lewis structures (in this case) because a single structure is insufficient to
describe the real structure.
Assignment:
Draw the Lewis structure and it’s resonance structure(s) for:
SO2
SO3
Lewis structures of Polyatomic Ions
Ammonium – NH4+
Miss Mayer
Chemistry 30
Drawing a Lewis structure for polyatomic ions has the same rules as with regular covalent
compounds but there are two more things that need to be done:
1) When counting your total valence electrons in step 1, you need to add or subtract
electrons based on the polyatomic ion's charge
Ex) Count electrons for the following:
NH4+1
PO4-3
SNC-1
2) Once you have completed drawing the Lewis structure, the whole structure must be
placed in square brackets and the charge is written outside of the brackets.
Ex:
Example 1: Draw the Lewis structure of chlorite, ClO2-1
Assignment: On a sheet of loose leaf, draw Lewis diagrams for the following polyatomic ions.
Note: Draw all relevant resonance structures if needed! (There are 4)