Which of the following molecules/ions has pyramidal shape?

Options

A. `SO_3`
B. `NH_4^+`
C. `PCl_3`
D. `H_2O`

Answer

To determine which of the given molecules/ions has a pyramidal shape, we need to analyze the hybridization and molecular geometry of each compound. Let's go through them step by step.

### Step 1: Analyze SO3 (Sulfur Trioxide)
1. **Valence Electrons**: Sulfur has 6 valence electrons.
2. **Bonding**: In SO3, sulfur forms three double bonds with oxygen atoms. Each double bond consists of one sigma bond and one pi bond.
3. **Sigma Bonds**: There are 3 sigma bonds and no lone pairs on the sulfur atom.
4. **Hybridization**: The hybridization can be calculated as:
\[ \text{Hybridization index} = \text{Number of sigma bonds} + \text{Number of lone pairs} = 3 + 0 = 3 \]
This corresponds to sp² hybridization.
5. **Shape**: The molecular shape is trigonal planar due to sp² hybridization.

**Conclusion for SO3**: Does not have a pyramidal shape.

### Step 2: Analyze NH4+ (Ammonium Ion)
1. **Valence Electrons**: Nitrogen has 5 valence electrons, and NH4+ means it has donated one electron, so it effectively has 4 valence electrons.
2. **Bonding**: NH4+ has 4 sigma bonds with hydrogen atoms and no lone pairs.
3. **Hybridization**:
\[ \text{Hybridization index} = 4 + 0 = 4 \]
This corresponds to sp³ hybridization.
4. **Shape**: The molecular shape is tetrahedral due to sp³ hybridization.

**Conclusion for NH4+**: Does not have a pyramidal shape.

### Step 3: Analyze PCl3 (Phosphorus Trichloride)
1. **Valence Electrons**: Phosphorus has 5 valence electrons.
2. **Bonding**: PCl3 has 3 sigma bonds with chlorine atoms and 1 lone pair on phosphorus.
3. **Hybridization**:
\[ \text{Hybridization index} = 3 + 1 = 4 \]
This corresponds to sp³ hybridization.
4. **Shape**: The presence of one lone pair and three bond pairs gives it a trigonal pyramidal shape.

**Conclusion for PCl3**: Has a pyramidal shape.

### Step 4: Analyze H2O (Water)
1. **Valence Electrons**: Oxygen has 6 valence electrons.
2. **Bonding**: H2O has 2 sigma bonds with hydrogen atoms and 2 lone pairs on oxygen.
3. **Hybridization**:
\[ \text{Hybridization index} = 2 + 2 = 4 \]
This corresponds to sp³ hybridization.
4. **Shape**: The molecular shape is bent or V-shaped due to the presence of 2 lone pairs.

**Conclusion for H2O**: Does not have a pyramidal shape.

### Final Conclusion
Among the four compounds analyzed, only **PCl3** has a trigonal pyramidal shape.

### Answer: PCl3

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