Chemistry · General chemistry I · Worked example
Draw the resonance structures of nitrate
Draw the Lewis structure of the nitrate ion, NO₃⁻, and its resonance structures.
Count the electrons
Nitrogen brings 5, each oxygen 6, and the negative charge adds 1.
Connect and complete the octets
Nitrogen goes in the center. Three single bonds use 6 electrons, and the other 18 give each oxygen three lone pairs. Nitrogen then has only 6 electrons.
Form one double bond
Move one lone pair from an oxygen into its bond to nitrogen. Nitrogen now has an octet.
Assign formal charges
Nitrogen: 5 − 0 − 8/2 = +1. The double-bonded oxygen: 6 − 4 − 2 = 0. Each single-bonded oxygen: 6 − 6 − 1 = −1. The total is +1 + 0 − 1 − 1 = −1, the charge of the ion.
Draw the resonance structures
The double bond could have been made to any of the three oxygens, so there are three equivalent structures. The real ion is their hybrid: three identical nitrogen–oxygen bonds, each with bond order 4/3, and each oxygen carrying −2/3 on average.
Result
Three equivalent structures, each with one double and two single nitrogen–oxygen bonds; formal charges +1 on nitrogen, 0 on the double-bonded oxygen and −1 on each single-bonded oxygen. The real ion is their hybrid.
Your turn
How many resonance structures does the carbonate ion, CO₃²⁻, have, and what is the carbon–oxygen bond order in the hybrid?
Show the answer and explanation
Three structures; bond order 4/3.
4 + 3(6) + 2 = 24 electrons, arranged as one double and two single carbon–oxygen bonds, with the double bond on any of the three oxygens. Averaging the three bonds gives (2 + 1 + 1)/3 = 4/3.
Keep exploring
In Lewis electrons & resonance, the two forms keep the same atoms and neighbors but move the double bond. The studio confirms consistent bookkeeping and the same −1 total charge for both.
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