Which of the following pairs does not represent resonance structures?

• A. $O_3$ structures: $O=O-O$ and $O-O=O$
• B. ${\mathrm{CH}}_3\mathrm{CH}=O$ and ${\mathrm{CH}}_2=\mathrm{CH}-\mathrm{OH}$
• C. $\mathrm{CO}^2-$ structures: $O=C-O^-$ and $O-C=O$
• D. ${\mathrm{NO}}_2^-$ structures: $O=N-O^-$ and $O-N=O$

Answer: (B)

Verified step by step guidance

1. Step 1: Understand what resonance structures are. Resonance structures are different Lewis structures for the same molecule that differ only in the placement of electrons (usually pi electrons or lone pairs), not in the arrangement of atoms. The actual molecule is a resonance hybrid of these structures.
2. Step 2: Examine each pair of structures to see if they have the same arrangement of atoms. If the atoms are connected differently (i.e., different connectivity), then they are not resonance structures but different compounds or isomers.
3. Step 3: For the given pairs, check the connectivity of atoms. For example, in the ozone (O\_3) structures, the atoms are connected in the same order (O-O-O), but the placement of double bonds and charges differ, so these are resonance structures.
4. Step 4: Look at the pair involving \( \mathrm{CH_3CHO} \) and \( \mathrm{CH_2=CH-OH} \). These have different connectivity: one is an aldehyde, the other is an alcohol with a double bond. Since the atoms are connected differently, these are not resonance structures.
5. Step 5: Confirm that the other pairs (carbonate ion \( \mathrm{CO_3^{2-}} \) and nitrite ion \( \mathrm{NO_2^-} \)) have the same atom connectivity but differ in electron placement, so they represent valid resonance structures.