Which of the following is (are) gerade (g) `MO's` ? .

Options

A. `sigma 2s`
B. `sigma 2pz`
C. `pi^(**)(2px)`
D. `sigma^(**)2s`

Answer

To determine which of the given molecular orbitals (MOs) are gerade (g), we need to understand the characteristics of gerade and ungerade (u) molecular orbitals.

### Step-by-Step Solution:

1. **Understanding Molecular Orbitals**:
- Molecular orbitals can be classified as either gerade (g) or ungerade (u) based on their symmetry properties.
- A gerade molecular orbital is symmetric about the center of inversion, meaning that if you rotate it 180 degrees about the center, it looks the same.

2. **Identifying the Characteristics of MOs**:
- Generally, sigma (σ) bonding molecular orbitals are gerade, while pi (π) molecular orbitals can be either gerade or ungerade depending on their specific designation (e.g., π or π*).
- The presence of a star (e.g., π*) indicates an antibonding orbital, which is typically ungerade.

3. **Analyzing the Given Options**:
- Let's analyze the options provided:
- **Option A: σ2s** - This is a sigma bonding orbital, which is gerade.
- **Option B: σ2p** - This is also a sigma bonding orbital, hence it is gerade.
- **Option C: π2p** - This is a pi bonding orbital, which is generally ungerade.
- **Option D: π*2p** - This is a pi antibonding orbital, which is ungerade.

4. **Conclusion**:
- Based on the analysis, the gerade molecular orbitals from the options provided are:
- **Option A (σ2s)** and **Option B (σ2p)**.

### Final Answer:
The gerade molecular orbitals are **Option A and Option B**.