Which species is planar ?

Options

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

Explanation

To determine which species among CO3^2-, SO3^2-, ClO3^-, and BF4^- is planar, we will calculate the hybridization and steric number for each species using the formula:

**Steric Number (SN) = (1/2) * [V + M + (Charge)]**

Where:
- V = number of valence electrons of the central atom
- M = number of monovalent atoms attached
- Charge = +1 for cations and -1 for anions

### Step-by-Step Solution:

1. **CO3^2- (Carbonate Ion)**
- Central atom: Carbon (C)
- Valence electrons (V): 4 (from carbon)
- Monovalent atoms (M): 3 (three oxygen atoms)
- Charge: -2 (since it is an anion)
- Calculation:
\[ SN = \frac{1}{2} \times [4 + 3 + (-2)] = \frac{1}{2} \times 5 = 2.5 \text{ (round to 3)} \]
- Hybridization: sp² (since steric number is 3)
- Geometry: Trigonal planar (3 bond pairs, no lone pairs)

2. **SO3^2- (Sulfite Ion)**
- Central atom: Sulfur (S)
- Valence electrons (V): 6 (from sulfur)
- Monovalent atoms (M): 3 (three oxygen atoms)
- Charge: -2 (since it is an anion)
- Calculation:
\[ SN = \frac{1}{2} \times [6 + 3 + (-2)] = \frac{1}{2} \times 7 = 3.5 \text{ (round to 4)} \]
- Hybridization: sp³ (since steric number is 4)
- Geometry: Tetrahedral, but with one lone pair, it is pyramidal (not planar).

3. **ClO3^- (Chlorate Ion)**
- Central atom: Chlorine (Cl)
- Valence electrons (V): 7 (from chlorine)
- Monovalent atoms (M): 3 (three oxygen atoms)
- Charge: -1 (since it is an anion)
- Calculation:
\[ SN = \frac{1}{2} \times [7 + 3 + (-1)] = \frac{1}{2} \times 9 = 4.5 \text{ (round to 4)} \]
- Hybridization: sp³ (since steric number is 4)
- Geometry: Tetrahedral, but with one lone pair, it is pyramidal (not planar).

4. **BF4^- (Tetrafluoroborate Ion)**
- Central atom: Boron (B)
- Valence electrons (V): 3 (from boron)
- Monovalent atoms (M): 4 (four fluorine atoms)
- Charge: -1 (since it is an anion)
- Calculation:
\[ SN = \frac{1}{2} \times [3 + 4 + (-1)] = \frac{1}{2} \times 6 = 3 \]
- Hybridization: sp³ (since steric number is 4)
- Geometry: Tetrahedral (4 bond pairs, no lone pairs, but not planar).

### Conclusion:
The only planar species among the given options is **CO3^2- (Carbonate Ion)**.