Question

lewis structures, vsepr, polarity, im forces
for each of the following molecules, draw the lewis structure (with resonance structures, if applicable), indicate the molecular shapes and bond angles, indicate molecular polarity (if any), and identify the major intermolecular force in each compound.

1. carbon tetrafluoride
2. $bf_3$
3. $nf_3$
4. $h_2cs$
5. carbonate ion

Explanation:

---

1) Carbon tetrafluoride ($\text{CF}_4$)

Step1: Calculate total valence electrons

C: 4, F: 7 each. Total = $4 + (4 \times 7) = 32$

Step2: Draw Lewis structure

Central C bonded to 4 F atoms; each F has 3 lone pairs. No resonance.

Step3: VSEPR shape & bond angle

4 bonding pairs, 0 lone pairs: tetrahedral. Bond angle = $109.5^\circ$

Step4: Molecular polarity

Symmetric shape; polar C-F bonds cancel. Nonpolar.

Step5: Major intermolecular force

Nonpolar molecule: London dispersion forces.

---

2) $\text{BF}_3$

Step1: Calculate total valence electrons

B: 3, F: 7 each. Total = $3 + (3 \times 7) = 24$

Step2: Draw Lewis structure

Central B bonded to 3 F atoms; each F has 3 lone pairs. B has incomplete octet, no resonance.

Step3: VSEPR shape & bond angle

3 bonding pairs, 0 lone pairs: trigonal planar. Bond angle = $120^\circ$

Step4: Molecular polarity

Symmetric shape; polar B-F bonds cancel. Nonpolar.

Step5: Major intermolecular force

Nonpolar molecule: London dispersion forces.

---

3) $\text{NF}_3$

Step1: Calculate total valence electrons

N: 5, F: 7 each. Total = $5 + (3 \times 7) = 26$

Step2: Draw Lewis structure

Central N bonded to 3 F atoms; N has 1 lone pair. Each F has 3 lone pairs. No resonance.

Step3: VSEPR shape & bond angle

3 bonding pairs, 1 lone pair: trigonal pyramidal. Bond angle ≈ $107^\circ$

Step4: Molecular polarity

Asymmetric shape; polar N-F bonds do not cancel. Polar.

Step5: Major intermolecular force

Polar molecule with no H-bonding: dipole-dipole forces.

---

4) $\text{H}_2\text{CS}$

Step1: Calculate total valence electrons

C: 4, H:1 each, S:6. Total = $4 + (2 \times 1) + 6 = 12$

Step2: Draw Lewis structure

Central C double-bonded to S, single-bonded to 2 H atoms. S has 2 lone pairs. No resonance.

Step3: VSEPR shape & bond angle

3 bonding regions, 0 lone pairs: trigonal planar. Bond angle ≈ $120^\circ$

Step4: Molecular polarity

Asymmetric shape; polar C-S and C-H bonds do not cancel. Polar.

Step5: Major intermolecular force

Polar molecule with no H-bonding: dipole-dipole forces.

---

5) Carbonate ion ($\text{CO}_3^{2-}$)

Step1: Calculate total valence electrons

C:4, O:6 each, +2 charge. Total = $4 + (3 \times 6) + 2 = 24$

Step2: Draw Lewis structure

Central C bonded to 3 O atoms; 1 double bond, 2 single bonds. 3 resonance structures exist. Each single-bonded O has 3 lone pairs, double-bonded O has 2 lone pairs.

Step3: VSEPR shape & bond angle

3 bonding regions, 0 lone pairs: trigonal planar. Bond angle = $120^\circ$ (resonance-averaged)

Step4: Molecular polarity

Symmetric resonance hybrid; polar C-O bonds cancel. Nonpolar.

Step5: Major intermolecular force

Ion; interacts via ion-ion forces (with other ions) or ion-dipole (with polar molecules).

Answer:

1. Carbon tetrafluoride ($\text{CF}_4$)

• Lewis Structure: Central C with 4 single bonds to F; each F has 3 lone pairs
• Shape: Tetrahedral; Bond angle: $109.5^\circ$
• Polarity: Nonpolar
• Major IM Force: London dispersion forces

1. $\text{BF}_3$

• Lewis Structure: Central B with 3 single bonds to F; each F has 3 lone pairs
• Shape: Trigonal planar; Bond angle: $120^\circ$
• Polarity: Nonpolar
• Major IM Force: London dispersion forces

1. $\text{NF}_3$

• Lewis Structure: Central N with 3 single bonds to F, 1 lone pair; each F has 3 lone pairs
• Shape: Trigonal pyramidal; Bond angle: $\approx 107^\circ$
• Polarity: Polar
• Major IM Force: Dipole-dipole forces

1. $\text{H}_2\text{CS}$

• Lewis Structure: Central C with double bond to S, 2 single bonds to H; S has 2 lone pairs
• Shape: Trigonal planar; Bond angle: $\approx 120^\circ$
• Polarity: Polar
• Major IM Force: Dipole-dipole forces

1. Carbonate ion ($\text{CO}_3^{2-}$)

• Lewis Structure: Central C with 1 double bond to O, 2 single bonds to O (3 resonance forms); single-bonded O have 3 lone pairs, double-bonded O has 2 lone pairs
• Shape: Trigonal planar; Bond angle: $120^\circ$
• Polarity: Nonpolar
• Major IM Force: Ion-ion (or ion-dipole) forces