The pair having similar geometry is

Options

A. `PCl_(3), NH_(4)^(+)`
B. `BeCl_(2), H_(2)O`
C. `CH_(4), CCl_(4)`
D. `IF_(5), PF_(5)`

Explanation

To determine which pairs of molecules have similar geometry, we will analyze the steric numbers of the given molecules and their corresponding geometries.

### Step-by-Step Solution:

1. **Identify the Molecules:**
The molecules we need to analyze are:
- PCl₃ (Phosphorus trichloride)
- NH₄⁺ (Ammonium ion)
- BeCl₂ (Beryllium dichloride)
- H₂O (Water)

2. **Calculate the Steric Number:**
The steric number can be calculated using the formula:
\[ \text{Steric Number} = \frac{1}{2} \left( \text{Valence Electrons of Central Atom} + \text{Monovalent Atoms} + \text{Cationic Charge} - \text{Anionic Charge} \right) \]

- **For PCl₃:**
- Valence electrons of phosphorus = 5
- Monovalent atoms (3 chlorine) = 3
- No charge = 0
\[ \text{Steric Number} = \frac{1}{2} (5 + 3 + 0) = \frac{1}{2} \times 8 = 4 \]

- **For NH₄⁺:**
- Valence electrons of nitrogen = 5
- Monovalent atoms (4 hydrogen) = 4
- Cationic charge = 1
\[ \text{Steric Number} = \frac{1}{2} (5 + 4 - 1) = \frac{1}{2} \times 8 = 4 \]

- **For BeCl₂:**
- Valence electrons of beryllium = 2
- Monovalent atoms (2 chlorine) = 2
- No charge = 0
\[ \text{Steric Number} = \frac{1}{2} (2 + 2 + 0) = \frac{1}{2} \times 4 = 2 \]

- **For H₂O:**
- Valence electrons of oxygen = 6
- Monovalent atoms (2 hydrogen) = 2
- No charge = 0
\[ \text{Steric Number} = \frac{1}{2} (6 + 2 + 0) = \frac{1}{2} \times 8 = 4 \]

3. **Determine the Geometry:**
- **PCl₃:** Steric number = 4 → Tetrahedral geometry
- **NH₄⁺:** Steric number = 4 → Tetrahedral geometry
- **BeCl₂:** Steric number = 2 → Linear geometry
- **H₂O:** Steric number = 4 → Tetrahedral geometry

4. **Identify Pairs with Similar Geometry:**
From the analysis, we can see:
- PCl₃ and NH₄⁺ both have tetrahedral geometry.
- H₂O also has tetrahedral geometry.

### Conclusion:
The pairs having similar geometry are **PCl₃ and NH₄⁺**, and **H₂O** also shares tetrahedral geometry with them.