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Resonance Structures
The resonance means shifting multiple bond or bonds between equivalent positions in a given
molecule. That means a single Lewis structure cannot adequately describe the single molecule.
In that case more than one Lewis structure are used describe the molecule. These structures are
known as resonance structures. There are, however, two requirements; (a) must have at least one
multiple bond, and (b) must have same two atoms in different location with the same
environments. Let us illustrate this with following example.
Example
Write the possible resonance structures for nitrate ion (NO3-).
Answer
Lewis dot symbols:
Step 1. From the above dot notations, count the total number of valence electrons. There is one N
atom and three O atoms in nitrate ion.
Total number of valence electrons = 5 (for N) + 6 (for O) + 6 (for O) + 6 (for O)
+ 1 (for 1 negative charges)
= 24 electrons
Step 2: All atoms obey octet rule.
Number of electrons needed to obey the rules = 8(for N) + 8 (for O) +8(for O)+ 8 (for O)
= 32 electrons.
Step 3. Number of electrons engaged in bonding = 32 electrons - 24 electrons = 8 electrons.
Step 4. Number of single bonds = 8 electrons / 2 electrons = 4 single bonds.
Step 5. Now place the C atom in the center and O atoms on any 3 sides.
You have 8 electrons (or 4 bonds) to be distributed. First place the single bond ( 2 dots) between
each O and N. You have placed 6 electrons so far. You have 2 more left; place them between
any one of the O and N. Now you have distributed all bonding electrons. Total you have 24
valence electrons and still you have 16 valence electrons to be distributed. Place required
number of electrons around each O atom to complete the octet.
