Problem 92 - Chemistry The Science in Context

Question

Use formal charges to determine which resonance form of each of the following ions is preferred: CNO"; NCO"; \(\mathrm{CON}^{-}\).

Step-by-Step Solution

Answer

Answer: The preferred resonance forms are: - CNO"": \(\mathrm{C-N}^{+}=O\) - NCO"": \(\mathrm{\text{-}N=C=O}\) - \(\mathrm{CON}^{-}\): Both \(\mathrm{O^{-}-C=N}\) and \(\mathrm{O=C-N^{-}}\) contribute equally

Step 1: Resonance Forms and Formal Charges Calculation

To begin, let's draw the resonance structures for each ion and find the formal charges of each atom for every structure. 1. CNO" - Resonance form 1: \(\mathrm{\text{-}C=N-O^{+}}\) - Formal charges: C: -1, N: 0, and O: +1 - Resonance form 2: \(\mathrm{\text{C-N}^{+}=O}\) - Formal charges: C: 0, N: +1, and O: 0 For this ion, resonance form 2 is preferred because it has fewer charges on the atoms. 2. NCO" - Resonance form 1: \(\mathrm{\text{-}N=C=O}\) - Formal charges: N: -1, C: 0, and O: 0 - Resonance form 2: \(\mathrm{N=C-O^{+}}\) - Formal charges: N: 0, C: 0, and O: +1 For this ion, resonance form 1 is preferred because it has fewer charges on the atoms. 3. \(\mathrm{CON}^{-}\) - Resonance form 1: \(\mathrm{O^{-}-C=N}\) - Formal charges: O: -1, C: 0, and N: 0 - Resonance form 2: \(\mathrm{O=C-N^{-}}\) - Formal charges: O: 0, C: 0, and N: -1 For this ion, both resonance forms are equivalent in terms of formal charges, so they are both preferred and contribute equally to the overall resonance hybrid. So, the preferred resonance forms are: - CNO": \(\mathrm{C-N}^{+}=O\) - NCO": \(\mathrm{\text{-}N=C=O}\) - \(\mathrm{CON}^{-}\): Both \(\mathrm{O^{-}-C=N}\) and \(\mathrm{O=C-N^{-}}\) contribute equally

Key Concepts

Formal Charges

Formal charges are a crucial concept in understanding molecular structures, especially when dealing with resonance structures. They serve as a book-keeping device that helps chemists determine the actual distribution of electrons within a molecule. In essence, a formal charge of an atom in a molecule can be found using the formula: \[ \text{Formal charge} = \text{Valence electrons (atom)} - \text{Non-bonding electrons} - \frac{1}{2} \times \text{Bonding electrons} \]This calculation helps to identify which resonance structure is more stable or preferable based on the arrangement of charges on the atoms involved.

• Atoms with formal charges close to zero usually indicate more stability.
• Negative formal charges are often more stable on electronegative elements like oxygen.

In the given ions, calculating and comparing the formal charges of different resonance forms aids in determining the most representative structure. The lesser the charge separation, the more stable the ion structure is perceived to be.

Chemical Bonding

Chemical bonding refers to the forces that hold atoms together in a molecule, and it plays a critical role in the formation of resonance structures. Understanding how atoms bond allows us to predict the possible resonance forms that a molecule might adopt. Bonds are typically classified into covalent, ionic, and metallic based on the nature of the electron interaction.

• Covalent bonding involves the sharing of electrons between two non-metals, forming stable molecules, as exhibited in the resonance forms of the given ions.
• Ionic bonds occur due to the transfer of electrons from one atom to another, generating charged ions.

In the context of resonance, the bonds between atoms can be delocalized across multiple structures, which enhances stability. Each resonance form represents a different arrangement of bonds and lone pairs of electrons, portraying the molecule as a blend or hybrid of all possible structures. This blending often leads to an intermediate state where bond lengths and energies are averaged.

Ion Stability

Ion stability is a concept tied closely to how consistently an ion can maintain its structure under certain conditions. Several factors contribute to ion stability, including the type of bonding, the presence of resonance, and the distribution of formal charges.

• Resonance stabilization occurs when electron density is delocalized over multiple structures, as seen in the resonance forms of ions like \(\mathrm{CON}^{-}\).
• Formal charge distribution plays a significant role; ions with minimized overall charge, symmetrical charge distribution, and charges on more electronegative atoms generally exhibit greater stability.

Thus, when evaluating resonance structures, one considers both the formal charge distribution and how resonance affects the actual electron configuration of the molecule. By understanding these principles, one can better predict which resonance forms are more favorable, as illustrated by the ion examples shared, where ions with distributed charges and greater resonance options are more stable.