`MO' s` are formed by the overlap of `A'O s` Two `AO's` combine to form two `MO's` called bonding molecular orbital `(BMO)` and antibonding molecular orbital `(ABMO)` Differnet `AO' s` of one atom combine with these `AO 's` of the second atom which have comparable energies and proper orienation Further, If the overlapping is head on, the `MO` is called 'pi' The `MO's` are filled with electrons following the same rules as followed for filling of atomic orbitals However the order of filling is not the same for all molecules or their ions Bond order is one of the most important parameter to compare a number of their characteristics
In which of the following pair both to `MO's` are gerade or ungerade ? .

Options

A. `sigma2s,pi2p_(x)`
B. `sigma^(**)2s,pi^(**)2p_(x)`
C. `sigma^(**)2s,pi2p_(x)`
D. `pi2p_(x),pi^(**)2p_(x)`

Answer

To determine which pair of molecular orbitals (MOs) are both gerade (symmetric) or ungerade (asymmetric), we need to analyze the characteristics of the given molecular orbitals based on their symmetry properties.

### Step-by-Step Solution:

1. **Understanding Molecular Orbitals**:
- Molecular orbitals can be classified as either gerade (symmetric) or ungerade (asymmetric).
- A molecular orbital is termed **gerade** if its wave function remains unchanged (symmetric) when reflected through a plane; it is termed **ungerade** if it changes sign (asymmetric).

2. **Identifying the Molecular Orbitals**:
- The question provides pairs of molecular orbitals. We need to analyze each pair to determine their symmetry.

3. **Analyzing Each Pair**:
- **Pair 1: σ2s and π2px**:
- σ2s is a bonding molecular orbital and is gerade.
- π2px is a bonding molecular orbital and is ungerade.
- **Conclusion**: One is gerade, one is ungerade.

- **Pair 2: σ*2s and π*2px**:
- σ*2s is an antibonding molecular orbital and is ungerade.
- π*2px is an antibonding molecular orbital and is gerade.
- **Conclusion**: One is ungerade, one is gerade.

- **Pair 3: σ*2p and π2px**:
- σ*2p is an antibonding molecular orbital and is ungerade.
- π2px is a bonding molecular orbital and is ungerade.
- **Conclusion**: Both are ungerade.

- **Pair 4: π2px and π*2px**:
- π2px is a bonding molecular orbital and is ungerade.
- π*2px is an antibonding molecular orbital and is gerade.
- **Conclusion**: One is ungerade, one is gerade.

4. **Final Conclusion**:
- The only pair where both molecular orbitals are either both gerade or both ungerade is **Pair 3: σ*2p and π2px**, as both are ungerade.

### Answer:
The correct answer is **Pair 3: σ*2p and π2px**, as both are ungerade.