Problem 99 - Chemistry Principles and Practice

Question

The molecule nitrosyl chloride, NOCl, has a skeleton structure of \(\mathrm{O}-\mathrm{N}-\mathrm{Cl}\). Two resonance forms can be written; write them both. Use the formal charge stability rules to predict which form is more stable.

Step-by-Step Solution

Answer

The more stable resonance structure is O-N=Cl.

Step 1: Determine Total Valence Electrons

Calculate the total number of valence electrons for NOCl. Nitrogen (N) has 5 valence electrons, oxygen (O) has 6, and chlorine (Cl) has 7. Total valence electrons = 5 (N) + 6 (O) + 7 (Cl) = 18 electrons.

Step 2: Draw Basic Lewis Structure

Arrange O, N, and Cl in a linear structure: O-N-Cl. Distribute the total valence electrons to satisfy the octet rule, starting by placing single bonds between the atoms (O-N and N-Cl). Use any remaining electrons to complete the octet for each atom.

Step 3: Draw First Resonance Structure

Assign a double bond between O and N: O=N-Cl. Distribute the remaining electrons around each atom to complete the octet.

Step 4: Draw Second Resonance Structure

Assign a double bond between N and Cl: O-N=Cl. Again, distribute the electrons to satisfy the octet rule for each atom.

Step 5: Calculate Formal Charges

For the first structure (O=N-Cl), the formal charges are calculated as: Formal charge (O) = 6 - (4 nonbonding + 2 bonding/2) = 0, Formal charge (N) = 5 - (0 nonbonding + 6 bonding/2) = +1, Formal charge (Cl) = 7 - (6 nonbonding + 2 bonding/2) = 0.

Step 6: Calculate Formal Charges for Second Structure

For the second structure (O-N=Cl), calculate formal charges: Formal charge of O = 6 - (6 nonbonding + 2 bonding/2) = -1, Formal charge of N = 5 - (2 nonbonding + 4 bonding/2) = 0, Formal charge of Cl = 7 - (4 nonbonding + 4 bonding/2) = 0.

Step 7: Analyze Stability Based on Formal Charges

Resonance structures are more stable when they minimize formal charges. In the first structure O=N-Cl, formal charges are +1 on N and 0 on the others, totaling +1 overall. In the second structure O-N=Cl, formal charges are -1 on O and 0 on the others, totaling -1 overall. The second structure, with the smallest overall formal charge, is more stable.

Key Concepts

Lewis Structures

Lewis structures are visual representations of the arrangement of atoms and the distribution of electrons in a molecule. They help in understanding how atoms bond and how electrons are shared or transferred in compounds. For the molecule nitrosyl chloride (NOCl), the skeletal structure is arranged as O-N-Cl.

When drawing a basic Lewis structure, start by connecting atoms with single bonds to satisfy the octet rule, ensuring that each atom is surrounded by eight electrons. In NOCl, this involves connecting oxygen to nitrogen and nitrogen to chlorine. Use the total number of valence electrons, which in this case is 18, to assign single bonds and lone pairs around atoms.

• Single bonds are depicted using a straight line between two atoms.
• Lone pairs are shown as dots around the chemical symbol.

Understanding Lewis structures is fundamental for visualizing molecules and making predictions about molecular shape, polarity, and reactivity.

Formal Charges

Formal charges offer insight into the electron distribution within the resonance structures of a molecule. Calculating formal charge helps identify the most stable resonance form. This stability is crucial as it determines the preferred electron arrangement in a molecule.

For each atom in a molecule, calculate the formal charge using the formula: \[\text{Formal charge} = \text{Valence electrons} - \text{(Nonbonding electrons + Bonding electrons/2)}\]

• Valence electrons are the electrons in the outermost shell of the neutral atom.
• Nonbonding electrons are those not involved in any bonds.
• Bonding electrons are shared between atoms in bonds (counted as half for each atom involved).

Using formal charges for nitrosyl chloride, we found that the second resonance structure (O-N=Cl) is more stable due to lower formal charges, specifically, [-1 on O, 0 on N and Cl] which deviates less from zero than the first structure's formal charges.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding and reactions. Familiarity with the number of valence electrons is pivotal when drawing Lewis structures and predicting molecular behavior.

For nitrosyl chloride (NOCl), calculate the total valence electrons to ensure that you distribute them accurately in the Lewis structure. Valence electrons for each element are:

• Nitrogen (N) has 5 valence electrons.
• Oxygen (O) has 6 valence electrons.
• Chlorine (Cl) has 7 valence electrons.

Adding these, we get a total of 18 valence electrons, which must be allocated among the atoms to fulfill the octet rule, often forming single or double bonds and adding lone pairs.
Valence electrons dictate the interactions between atoms, influencing bond formation and the stability of a molecule.