Which of the following statements is not correct from the point of view of molecular orbital theory ?

Options

A. `Be_(2)` is not a stable molecule
B. `He_(2)` is not stable but `He_(2)^(+)` is expected to exist
C. Bond strength of `N_(2)` is maximum amongst the homonuclear diatomic molecules
D. The order of energies of molecular orbitals in `F_(2)` molecule is `pi_(2p_(x)) = pi_(2p_(y)) lt sigma_(2p_(z))`

Answer

To determine which statement is not correct from the point of view of molecular orbital theory, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Molecular Orbital Theory**:
- Molecular Orbital (MO) theory explains the behavior of electrons in molecules. Electrons occupy molecular orbitals, which can be bonding (lower energy) or antibonding (higher energy).
- The bond order can be calculated using the formula:
\[ \text{Bond Order} = \frac{(N_b - N_a)}{2} \]
where \(N_b\) is the number of electrons in bonding orbitals and \(N_a\) is the number of electrons in antibonding orbitals.

2. **Analyzing the Statements**:
- **Option A**: "Beryllium (Be2) is not a stable molecule."
- For Be2, the electronic configuration is \( (1\sigma_g)^2 (1\sigma_u^*)^2 \).
- Here, \(N_b = 2\) and \(N_a = 2\), so the bond order is:
\[ \text{Bond Order} = \frac{(2 - 2)}{2} = 0 \]
- A bond order of 0 indicates that Be2 is indeed not stable. This statement is correct.

- **Option B**: "He2 is not stable."
- For He2, the electronic configuration is \( (1\sigma_g)^2 (1\sigma_u^*)^2 \).
- Similarly, the bond order is:
\[ \text{Bond Order} = \frac{(2 - 2)}{2} = 0 \]
- Therefore, He2 is also not stable. This statement is correct.

- **Option C**: "He2+ is expected to exist."
- For He2+, one electron is removed from the antibonding orbital:
\[ \text{Bond Order} = \frac{(2 - 1)}{2} = \frac{1}{2} \]
- A bond order of 0.5 indicates that He2+ is expected to exist. This statement is correct.

- **Option D**: "The order of energy is such that sigma energy is greater than pi."
- In molecular orbital theory, for diatomic molecules, sigma bonds (σ) are lower in energy than pi bonds (π) for elements in the second period (like O2 and F2). Therefore, this statement is incorrect.

3. **Conclusion**:
- The statement that is not correct from the point of view of molecular orbital theory is **Option D**: "The order of energy is such that sigma energy is greater than pi."

### Final Answer:
The statement that is not correct is **Option D**.