Challenge 1
Question:
Which of the following pairs of structures represent resonance structures?
Ready? Let’s dive in!
What are resonance structures?
Resonance structures are different representations of the same molecule. The key idea is that only the electrons move—specifically the π-electrons (from double or triple bonds) or non-bonding electrons (lone pairs). The atoms do not move, and the connections between them do not change.
Checklist for resonance forms
For two structures to be resonance forms, they must meet all of the following conditions:
✅ Same molecular formula
The number and types of atoms must be exactly the same in both structures.
✅ Same atom connectivity (no single bonds broken or formed)
The atoms must remain connected in the same way, with no rearranging, breaking, or forming of single bonds.
✅ Same net charge
The overall charge of the molecule (or ion) must stay the same in all resonance forms.
✅ Obey the octet rule
The molecule must adhere to basic bonding rules. For example:
- Second-row elements like carbon, nitrogen, and oxygen cannot have more than 8 electrons in their valence shell.
- Hydrogen can only have 2 electrons.
How to solve this
Now that we know what resonance structures are, let’s analyze the given pairs step by step using the checklist. The easiest way to identify resonance forms is by elimination—if a pair of structures violates even one rule, they are not resonance forms.
Step 1: Do they have the same molecular formula?
- Pairs A, B, and D have the same molecular formula.
- In Pair C, the second structure has an extra hydrogen atom, so the structures in Pair C are not resonance forms.
Let’s move on to the next step with the remaining pairs (A, B, and D).
Step 2: Are the atoms connected the same way?
- In Pair B, the molecular skeletons are different. The atoms are rearranged, which makes them constitutional isomers, not resonance forms.
- In Pair D, a single bond appears in the second structure that isn’t present in the first. This also makes them constitutional isomers, not resonance forms.
At this point, only Pair A remains. Let’s confirm it meets the other conditions.
Step 3: Do they have the same net charge?
Both structures in Pair A have the same net charge of zero.
Good to go! Let’s move on to the final step.
Step 4: Do they obey the octet rule?
To check this, we’ve included the lone pairs on nitrogen and oxygen for clarity.
In Pair A, all elements follow the octet rule:
- Carbon, nitrogen, and oxygen have no more than 8 electrons in their valence shells.
- Hydrogen has only 2 electrons.
🏆 Conclusion
And there we have it! The answer to this question is Pair A.
Challenge 2
Question:
Which of the following pairs of structures represent resonance structures?
Ready? Let’s dive in!
Step 1: Do they have the same molecular formula?
The structures in Pairs A, B, C, and D all have the same molecular formula.
So, we can move on to the next step!
Step 2: Are the atoms connected the same way?
In Pairs A, B, and C, the atoms are connected in the same way. No single bonds are formed or broken, so they pass this step.
At first glance, Pair D might look like it could contain resonance structures, but there’s a problem: the connectivity rule is violated. Let’s clarify by drawing in the hydrogen atoms.
For example, the carbons highlighted in Pair D are bonded to a different number of hydrogen atoms. This violates the connectivity rule, so we can rule out Pair D.
Step 3: Do they have the same net charge?
- Pair A has the same net charge of zero.
- Pair B also has the same net charge of zero.
- Pair C has a +1 net charge.
All three pairs—A, B, and C—pass this step. Let’s move on to the final check!
Step 4: Do they obey the octet rule?
- Pair B: All atoms, including carbon, nitrogen, and oxygen, have a complete octet, and no bonding rules are broken. This pair satisfies all resonance criteria.
- Pair C: Carbon lacks a full octet in both structures, but the Lewis structures are valid and follow the rules of resonance.
- Pair A: At first glance, this pair seems to follow resonance rules because the two structures differ only in the placement of the π-electrons in the C=O double bond. However, let’s take a closer look at the second structure.
In the second structure of Pair A, oxygen has only 6 electrons in its valence shell and carries a positive charge, while carbon has a full octet and a negative charge. Here’s the issue:
Electronegative atoms like oxygen or nitrogen strongly prefer a full octet, and it is unfavorable for them to carry a positive charge with fewer than 8 electrons.
On the other hand, carbon is less electronegative and is more stable when it lacks an octet, such as in the case of a carbocation.
Because of this, the second structure in Pair A is considered insignificant as a resonance contributor. Pair A does not qualify as valid resonance forms.
🏆 Conclusion
Pairs B and C are valid resonance forms.
