Based on formal charges, which of the three Lewis structures shown for CH2ON– is the dominant Lewis structure?

10A

Question

Based on formal charges, which of the three Lewis structures shown for CH2ON– is the dominant Lewis structure?

10A

BlackTom AI · Accepted Answer

Step through each option by applying formal charge accounting and octet considerations to CH2ON–.

Option 1 (Structure 1): In this structure, the carbon is bonded to two hydrogens, to an oxygen, and to a nitrogen, giving it four single bonds and an octet. The oxygen is bonded to the carbon by a single bond and bears three lone pairs, which gives it a formal charge of -1 (valence electrons 6 minus (nonbonding 6 + bonding electrons/2 = 1) = -1). The nitrogen has two hydrogens and a single bond to carbon, plus its lone pairs; its formal charge can be evaluated as near neutral or slightly negative depending on the exact lone-pair distribution, but in typical CH2ON– resonance this arrangement keeps the overall charge balanced with the single negative on oxygen, which is the most electronegative atom present. Overall, Structure 1 satisfies octets for all atoms and places the negative charge on the most appropriate, most electronegative element (oxygen), consistent with the anionic charge of the species.

Option 2 (Structure 2): This structure shows a C=O double bond and a N–H single bond with a lone-pair-bearing N. If a C=O double bond is present, carbon would typically carry fewer formal charges, but introducing a C=N or rearranged lone pairs can place positive or less favorable charges on carbon or nitrogen. In many depictions of CH2ON–, placing a double bond to carbon or altering the N center tends to create unfavorable formal charges on carbon or require charge separation that is less favorable for the overall anion. The result is a structure that either increases separation of charge or violates octet stability for one of the atoms, making it a poorer major contributor.

Option 3 (Structure 3): This arrangement places the carbonyl (C=O) with oxygen double-bonded to carbon, while the N–H fragment is positioned differently with a lone-pair distribution on nitrogen. Similar to Option 2, reassigning the bond order and lone pairs in this way tends to place a formal charge on carbon or nitrogen that is less consistent with the overall -1 charge and/or disrupts octet stability. The net outcome is a less favorable formal-charge distribution and/or less favorable octet satisfaction compared to Structure 1.

Option 4 (All three structures are equivalent): This would imply identical formal-charge distributions and identical octet satisfaction across all three drawings. In reality, the three Lewis structures show different bond patterns (single vs. double bonds, different placements of lone pairs) which lead to different formal charges on atoms and different octet satisfaction. Therefore, they are not equivalent contributors.

In summary, Structure 1 best satisfies both octet rules for all atoms and places the negative formal charge on the most electronegative element (oxygen) in a way that aligns with CH2ON–’s overall -1 charge, making it the dominant resonance form. The other structures introduce less favorable charge distributions or violate octet stability, making them less significant contributors.

CHEM 1210 AU2025 (15738) Exam 4- Requires Respondus LockDown Browser

Based on formal charges, which of the three Lewis structures shown for CH2ON– is the dominant Lewis structure? 10A

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