Which of the following statements is not correct from the viewpoint 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 `E(sigma2s)ltE(sigma^ast2s)ltE(2pip_x)=E(pi2p_y)ltE(sigma2p_z)ltE(pi^ast2p_x)=E(pi^ast2p_y)ltE(sigma^ast2p_z)`

Answer

To determine which of the statements is not correct from the viewpoint of molecular orbital theory (MOT), we will analyze each option one by one.

### Step-by-Step Solution:

1. **Option A: BE2 is not a stable molecule.**
- In BE2, the total number of electrons is 8. According to MOT, the electronic configuration is:
- σ1s² σ*1s² σ2s² σ*2s²
- The bond order is calculated as:
\[ \text{Bond Order} = \frac{N_b - N_a}{2} = \frac{0 - 4}{2} = 0 \]
- A bond order of 0 indicates that BE2 is not a stable molecule. **This statement is correct.**

2. **Option B: He2 is not stable, but He2+ is expected to exist.**
- In He2, the total number of electrons is 4. The electronic configuration is:
- σ1s² σ*1s²
- The bond order is:
\[ \text{Bond Order} = \frac{2 - 2}{2} = 0 \]
- This means He2 is not stable. For He2+, the total number of electrons is 3, and the configuration is:
- σ1s² σ*1s¹
- The bond order is:
\[ \text{Bond Order} = \frac{2 - 1}{2} = 0.5 \]
- A bond order of 0.5 indicates that He2+ can exist. **This statement is correct.**

3. **Option C: The bond strength of N2 is maximum among the homonuclear diatomic molecules.**
- In N2, the total number of electrons is 14. The electronic configuration is:
- σ1s² σ*1s² σ2s² σ*2s² π2p² (2 electrons in π2p) σ2p²
- The bond order is:
\[ \text{Bond Order} = \frac{10 - 4}{2} = 3 \]
- N2 has a bond order of 3, which is indeed the maximum bond order among homonuclear diatomic molecules. **This statement is correct.**

4. **Option D: The order of energies of molecular orbitals in F2 molecule is...**
- In F2, the total number of electrons is 18. The correct order of molecular orbitals is:
- σ1s² σ*1s² σ2s² σ*2s² σ2p² (2 electrons in π2p) π2p² σ2p²
- The order of energies is:
- σ1s < σ*1s < σ2s < σ*2s < π2p < σ2p < π*2p < σ*2p
- The provided order in the option is incorrect. **This statement is incorrect.**

### Conclusion:
The statement that is not correct from the viewpoint of molecular orbital theory is **Option D**.