Problem 116 - Chemistry: An Atoms-Focused Approach

Question

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

Step-by-Step Solution

Answer

Answer: The preferred resonance forms are: 1. CNO⁻: C=N=O 2. NCO⁻: N≡C-O 3. CON⁻: C=O-N

Step 1: Formal Charge Calculation

Formal charge can be calculated using the formula: Formal Charge = \((Valence~e^{-}) - (Nonbonding~e^{-}) - \frac{Bonding~e^{-}}{2}\) Now let's determine the valence electrons, nonbonding electrons, and bonding electrons for each atom in different resonance forms. For \(\mathrm{CNO}^-\): 1. Valence electrons: C (4), N (5), O (6) 2. Nonbonding electrons: Two structures are possible and nonbonding electrons will differ based on the location of the triple or double bond. 3. Bonding electrons: Two structures possible: C=N=O and C≡N-O For \(\mathrm{NCO}^-\): 1. Valence electrons: N (5), C (4), O (6) 2. Nonbonding electrons: Two structures are possible and nonbonding electrons will differ based on the location of the triple or double bond. 3. Bonding electrons: Two structures possible: N≡C-O and N=C=O For \(\mathrm{CON}^-\): 1. Valence electrons: C (4), O (6), N (5) 2. Nonbonding electrons: Two structures are possible and nonbonding electrons will differ based on the location of the triple or double bond. 3. Bonding electrons: Two structures possible: C=O-N and C-O≡N

Step 2: Determine Formal Charges

Now we will use the formula to calculate the formal charges for each atom in the resonance forms and determine which structures have minimized formal charges. 1. \(\mathrm{CNO}^-\): - C=N=O: Formal charges are C=0, N=-1, and O=0. - C≡N-O: Formal charges are C=-1, N=0, and O=0. 2. \(\mathrm{NCO}^-\): - N≡C-O: Formal charges are N=0, C=-1, and O=0. - N=C=O: Formal charges are N=+1, C=0, and O=-1. 3. \(\mathrm{CON}^-\): - C=O-N: Formal charges are C=-1, O=0, and N=0. - C-O≡N: Formal charges are C=0, O=+1, and N=-1.

Step 3: Choose the Preferred Resonance Forms

Now let's choose the preferred resonance forms based on minimized formal charges. 1. \(\mathrm{CNO}^-\): The preferred resonance form is C=N=O, as it has minimized formal charges with a single negative charge on the N atom and no positive charges. 2. \(\mathrm{NCO}^-\): The preferred resonance form is N≡C-O due to a single negative charge on the C atom and no positive charges. 3. \(\mathrm{CON}^-\): The preferred resonance form is C=O-N, as it has minimized formal charges with a single negative charge on the C atom and no positive charges.

Key Concepts

Resonance Forms

Resonance forms are different ways of drawing a molecule or ion by placing electrons in various positions. These forms do not depict different molecules but rather the same molecule with alternate electron arrangements. Resonance is important because it shows the delocalization of electrons, which contributes to the molecule's stability. For ions like

• the form with minimized formal charges is often the most stable.

Understanding resonance helps predict chemical behavior. When evaluating resonance structures for ions like the the position of multiple bonds and lone pairs can significantly affect the structure's stability.

Valence Electrons

Valence electrons are the outermost electrons of an atom that participate in bonding. They are crucial because they determine how atoms interact to form molecules. For molecules like

• Common elements involved include Carbon (with 4 valence electrons), Nitrogen (with 5), and Oxygen (with 6).

Valence electrons dictate the number of bonds an atom can form. For example, carbon forms four bonds to fill its valence shell. Accurately counting valence electrons helps in drawing the possible resonance forms without error.

Bonding Electrons

Bonding electrons are the electrons shared between atoms to form covalent bonds. These electrons hold atoms together within a molecule. When identifying bonding electrons, observe:

• Single bonds share 2 electrons.
• Double bonds share 4 electrons.
• Triple bonds share 6 electrons.

These shared electrons are distributed as bonded pairs in the Lewis structure. Different resonance forms might rearrange these pairs to present various possible structures. Calculating bonding electrons allows you to assess the formal charge by following the formula: where half of the bonding electrons are counted per atom.

Nonbonding Electrons

Nonbonding electrons, also known as lone pairs, are electrons that do not participate in bonding. They are located on a single atom and not shared with others. Nonbonding electrons significantly influence molecular shape and polarity.

• These electrons can often be rearranged in different resonance forms, impacting the atom's formal charge.
• While they do not contribute to bonding, nonbonding electrons play a critical role in defining the chemical properties and reactivity of the molecule.

Since they affect the overall charge distribution within a molecule, counting lone pairs accurately is crucial when evaluating resonance structures and determining the dominant form.