which of the following is not tetrahedral?

Options

A. `BF_(4)^(-)`
B. `NH_(4)^(+)`
C. `CO_(3)^(2-)`
D. `SO_(4)^(2-)`

Explanation

To determine which of the given options is not tetrahedral, we need to analyze the hybridization and steric numbers of each compound. The steric number can be calculated using the formula:

\[ \text{Steric Number} = \frac{1}{2} \left( \text{Number of valence electrons of the central atom} + \text{Number of monovalent atoms} - \text{Cationic charge} + \text{Anionic charge} \right) \]

Let's evaluate each option step by step.

### Step 1: Analyze Br4⁻

1. **Identify the central atom**: Bromine (Br).
2. **Valence electrons of Br**: 7 (Bromine has 7 valence electrons).
3. **Monovalent atoms**: 4 Chlorine (Cl) atoms.
4. **Cationic charge**: 0 (no positive charge).
5. **Anionic charge**: 1 (the overall charge is -1).

**Calculation**:
\[ \text{Steric Number} = \frac{1}{2} \left( 7 + 4 - 0 + 1 \right) = \frac{1}{2} \times 12 = 6 \]

This indicates an octahedral shape, not tetrahedral.

### Step 2: Analyze NH4⁺

1. **Identify the central atom**: Nitrogen (N).
2. **Valence electrons of N**: 5.
3. **Monovalent atoms**: 4 Hydrogen (H) atoms.
4. **Cationic charge**: 1 (the overall charge is +1).
5. **Anionic charge**: 0.

**Calculation**:
\[ \text{Steric Number} = \frac{1}{2} \left( 5 + 4 - 1 + 0 \right) = \frac{1}{2} \times 8 = 4 \]

This indicates an sp³ hybridization, which corresponds to a tetrahedral shape.

### Step 3: Analyze CO3²⁻

1. **Identify the central atom**: Carbon (C).
2. **Valence electrons of C**: 4.
3. **Monovalent atoms**: 0 (the oxygens are divalent).
4. **Cationic charge**: 0.
5. **Anionic charge**: 2.

**Calculation**:
\[ \text{Steric Number} = \frac{1}{2} \left( 4 + 0 - 0 + 2 \right) = \frac{1}{2} \times 6 = 3 \]

This indicates an sp² hybridization, which corresponds to a trigonal planar shape, not tetrahedral.

### Step 4: Analyze SO4²⁻

1. **Identify the central atom**: Sulfur (S).
2. **Valence electrons of S**: 6.
3. **Monovalent atoms**: 0.
4. **Cationic charge**: 0.
5. **Anionic charge**: 2.

**Calculation**:
\[ \text{Steric Number} = \frac{1}{2} \left( 6 + 0 - 0 + 2 \right) = \frac{1}{2} \times 8 = 4 \]

This indicates an sp³ hybridization, which corresponds to a tetrahedral shape.

### Conclusion

From the analysis, we find that:
- Br4⁻ is octahedral (not tetrahedral).
- NH4⁺ is tetrahedral.
- CO3²⁻ is trigonal planar (not tetrahedral).
- SO4²⁻ is tetrahedral.

Thus, the correct answer to the question "which of the following is not tetrahedral?" is **CO3²⁻**.