To determine the shapes of molecules using the Valence Shell Electron Pair Repulsion (VSEPR) theory, follow these steps:
a. PF3 (Phosphorus Trifluoride)
1. Determine the central atom: Phosphorus (P).
2. Count the total number of valence electrons: P has 5, and each F has 7, so 5 + (3 × 7) = 26.
3. Subtract the number of electrons used in bonds: 3 bonds × 2 electrons = 6, so 26 – 6 = 20.
4. Distribute the remaining electrons as lone pairs: 20 ÷ 2 = 10 lone pairs.
5. Phosphorus has 3 bonding pairs and 1 lone pair.
6. The electron pair geometry is tetrahedral, but the molecular shape is trigonal pyramidal due to the lone pair.
b. CCl4 (Carbon Tetrachloride)
1. Determine the central atom: Carbon (C).
2. Count the total number of valence electrons: C has 4, and each Cl has 7, so 4 + (4 × 7) = 32.
3. Subtract the number of electrons used in bonds: 4 bonds × 2 electrons = 8, so 32 – 8 = 24.
4. Distribute the remaining electrons as lone pairs: 24 ÷ 2 = 12 lone pairs.
5. Carbon has 4 bonding pairs and no lone pairs.
6. The electron pair geometry and molecular shape are both tetrahedral.
c. CO3^2- (Carbonate Ion)
1. Determine the central atom: Carbon (C).
2. Count the total number of valence electrons: C has 4, each O has 6, and there are 2 extra electrons due to the charge, so 4 + (3 × 6) + 2 = 24.
3. Subtract the number of electrons used in bonds: 3 bonds × 2 electrons = 6, so 24 – 6 = 18.
4. Distribute the remaining electrons as lone pairs: 18 ÷ 2 = 9 lone pairs.
5. Carbon has 3 bonding pairs and no lone pairs.
6. The electron pair geometry is trigonal planar, and the molecular shape is also trigonal planar.
d. SO4^2- (Sulfate Ion)
1. Determine the central atom: Sulfur (S).
2. Count the total number of valence electrons: S has 6, each O has 6, and there are 2 extra electrons due to the charge, so 6 + (4 × 6) + 2 = 32.
3. Subtract the number of electrons used in bonds: 4 bonds × 2 electrons = 8, so 32 – 8 = 24.
4. Distribute the remaining electrons as lone pairs: 24 ÷ 2 = 12 lone pairs.
5. Sulfur has 4 bonding pairs and no lone pairs.
6. The electron pair geometry and molecular shape are both tetrahedral.