Question

part a - lewis structures, resonance & formal charges

1. draw the lewis structure for \\(\text{so}_2\\).

a. provide all valid resonance structures.
b. assign formal charges.
c. use vsepr to predict the molecular shape.

1. draw the lewis structure of \\(\text{no}_2^-\\).

a. show resonance structures.
b. determine the formal charge on each atom.
c. predict the molecular geometry using vsepr.

Explanation:

Calculate total valence electrons

For Question 1 (\(\text{SO}_2\)):
Using the Chalcogens knowledge point, both sulfur (\(\text{S}\)) and oxygen (\(\text{O}\)) have 6 valence electrons.

$$ \text{Total } e^- = 6 + 2(6) = 18 $$

For Question 2 (\(\text{NO}_2^-\)):
Nitrogen (\(\text{N}\)) has 5 valence electrons, oxygen (\(\text{O}\)) has 6, and the negative charge adds 1.

$$ \text{Total } e^- = 5 + 2(6) + 1 = 18 $$

Determine Lewis and resonance structures

For \(\text{SO}_2\):
Central \(\text{S}\) forms one single bond and one double bond to satisfy octets with 18 electrons.
Resonance structures:

$$ \text{O}=\ddot{\text{S}}-\ddot{\text{O}}: \longleftrightarrow :\ddot{\text{O}}-\ddot{\text{S}}=\text{O} $$

For \(\text{NO}_2^-\):
Central \(\text{N}\) forms one single bond and one double bond.
Resonance structures:

$$ [\text{O}=\ddot{\text{N}}-\ddot{\text{O}}:]^- \longleftrightarrow [:\ddot{\text{O}}-\ddot{\text{N}}=\text{O}]^- $$

Calculate formal charges

Formal charge formula:

$$ \text{FC} = \text{Valence } e^- - \text{Non-bonding } e^- - \frac{1}{2}(\text{Bonding } e^-) $$

For \(\text{SO}_2\) (left resonance structure):

• Double-bonded \(\text{O}\): \(6 - 4 - 2 = 0\)
• Central \(\text{S}\): \(6 - 2 - 3 = +1\)
• Single-bonded \(\text{O}\): \(6 - 6 - 1 = -1\)

For \(\text{NO}_2^-\) (left resonance structure):

• Double-bonded \(\text{O}\): \(6 - 4 - 2 = 0\)
• Central \(\text{N}\): \(5 - 2 - 3 = 0\)
• Single-bonded \(\text{O}\): \(6 - 6 - 1 = -1\)

Predict molecular geometry using VSEPR

For \(\text{SO}_2\):

• Steric number = 3 (2 bonding groups + 1 lone pair on \(\text{S}\)).
• Electron-pair geometry: Trigonal planar.
• Molecular shape: Bent (angle \(< 120^\circ\)).

For \(\text{NO}_2^-\):

• Steric number = 3 (2 bonding groups + 1 lone pair on \(\text{N}\)).
• Electron-pair geometry: Trigonal planar.
• Molecular geometry: Bent (angle \(< 120^\circ\)).

Answer:

Question 1

a. Resonance Structures for \(\text{SO}_2\)

The two major resonance structures are:

$$ \ddot{\text{O}}=\ddot{\text{S}}-\ddot{\text{O}}:\quad \longleftrightarrow \quad :\ddot{\text{O}}-\ddot{\text{S}}=\ddot{\text{O}} $$

b. Formal Charges

For the structure \(\text{O}_{(1)}=\text{S}-\text{O}_{(2)}\):

• Double-bonded oxygen (\(\text{O}_{(1)}\)): \(0\)
• Central sulfur (\(\text{S}\)): \(+1\)
• Single-bonded oxygen (\(\text{O}_{(2)}\)): \(-1\)

c. VSEPR Molecular Shape

• Electron-pair Geometry: Trigonal planar
• Molecular Shape: Bent (bond angle \(\approx 119^\circ\))

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Question 2

a. Resonance Structures for \(\text{NO}_2^-\)

The two major resonance structures are:

$$ [\ddot{\text{O}}=\ddot{\text{N}}-\ddot{\text{O}}:]^-\quad \longleftrightarrow \quad [:\ddot{\text{O}}-\ddot{\text{N}}=\ddot{\text{O}}]^- $$

b. Formal Charges

For the structure \([\text{O}_{(1)}=\text{N}-\text{O}_{(2)}]^-\):

• Double-bonded oxygen (\(\text{O}_{(1)}\)): \(0\)
• Central nitrogen (\(\text{N}\)): \(0\)
• Single-bonded oxygen (\(\text{O}_{(2)}\)): \(-1\)

c. VSEPR Molecular Geometry

• Electron-pair Geometry: Trigonal planar
• Molecular Geometry: Bent (bond angle \(\approx 115^\circ\))