How many of the following species will not show pyramidal geometry?
`CO_3^(2-),SO_3^(2-),NO_3^-,PCl_3`

Answer

To determine how many of the given species will not show pyramidal geometry, we need to analyze each species based on their molecular geometry and steric numbers. Pyramidal geometry typically occurs when there are three sigma bonds and one lone pair on the central atom, leading to a steric number of 4.

Let's analyze each species one by one:

1. **CO₃²⁻ (Carbonate ion)**:
- Central atom: Carbon (C)
- Bonding: Carbon forms three sigma bonds with three oxygen atoms.
- Hybridization: The steric number is 3 (3 sigma bonds, 0 lone pairs).
- Geometry: Trigonal planar.
- Conclusion: **Does not show pyramidal geometry.**

2. **SO₃²⁻ (Sulfite ion)**:
- Central atom: Sulfur (S)
- Bonding: Sulfur forms three sigma bonds (two with oxygen atoms and one with a negatively charged oxygen).
- Lone pairs: There is one lone pair on sulfur.
- Hybridization: The steric number is 4 (3 sigma bonds + 1 lone pair).
- Geometry: Pyramidal.
- Conclusion: **Shows pyramidal geometry.**

3. **NO₃⁻ (Nitrate ion)**:
- Central atom: Nitrogen (N)
- Bonding: Nitrogen forms three sigma bonds with three oxygen atoms.
- Lone pairs: There are no lone pairs on nitrogen.
- Hybridization: The steric number is 3 (3 sigma bonds, 0 lone pairs).
- Geometry: Trigonal planar.
- Conclusion: **Does not show pyramidal geometry.**

4. **PCl₃ (Phosphorus trichloride)**:
- Central atom: Phosphorus (P)
- Bonding: Phosphorus forms three sigma bonds with three chlorine atoms.
- Lone pairs: There is one lone pair on phosphorus.
- Hybridization: The steric number is 4 (3 sigma bonds + 1 lone pair).
- Geometry: Pyramidal.
- Conclusion: **Shows pyramidal geometry.**

Now, let's summarize the findings:

- **CO₃²⁻**: Does not show pyramidal geometry.
- **SO₃²⁻**: Shows pyramidal geometry.
- **NO₃⁻**: Does not show pyramidal geometry.
- **PCl₃**: Shows pyramidal geometry.

**Final Count**: The species that do not show pyramidal geometry are **CO₃²⁻** and **NO₃⁻**. Therefore, the answer is **2 species**.