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 statement is not correct from the viewpoint of molecular orbital theory (MOT), we will analyze the statements one by one based on the principles of MOT.

### Step 1: Understand the Bond Order Calculation
The bond order is calculated using the formula:
\[ \text{Bond Order} = \frac{(\text{Number of bonding electrons}) - (\text{Number of antibonding electrons})}{2} \]
A bond order of 0 indicates that the molecule is unstable and does not exist.

### Step 2: Analyze Option 1 - Beryllium (Be2)
- For Be2, the electronic configuration in molecular orbitals is:
\[ \sigma_{1s}^2 \sigma_{1s}^*^2 \sigma_{2s}^2 \sigma_{2s}^*^2 \]
- Bond order calculation:
- Bonding electrons = 4 (from σ1s and σ2s)
- Antibonding electrons = 4 (from σ1s* and σ2s*)
\[ \text{Bond Order} = \frac{4 - 4}{2} = 0 \]
- Conclusion: Be2 is not stable and does not exist. This statement is correct.

### Step 3: Analyze Option 2 - Helium (He2)
- For He2, the electronic configuration is:
\[ \sigma_{1s}^2 \sigma_{1s}^*^2 \]
- Bond order calculation:
- Bonding electrons = 2
- Antibonding electrons = 2
\[ \text{Bond Order} = \frac{2 - 2}{2} = 0 \]
- Conclusion: He2 is not stable and does not exist. This statement is correct.

### Step 4: Analyze Option 3 - Bond Strength of Nitrogen (N2)
- For N2, the electronic configuration is:
\[ \sigma_{1s}^2 \sigma_{1s}^*^2 \sigma_{2s}^2 \sigma_{2s}^*^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \]
- Bond order calculation:
- Bonding electrons = 10
- Antibonding electrons = 4
\[ \text{Bond Order} = \frac{10 - 4}{2} = 3 \]
- Conclusion: N2 has the highest bond strength among diatomic molecules. This statement is correct.

### Step 5: Analyze Option 4 - Order of Molecular Orbitals for Fluorine (F2)
- For F2, the electronic configuration is:
\[ \sigma_{1s}^2 \sigma_{1s}^*^2 \sigma_{2s}^2 \sigma_{2s}^*^2 \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \]
- The correct order of molecular orbitals for F2 should be:
\[ \sigma_{2p_z} < \pi_{2p_x} = \pi_{2p_y} < \sigma_{2p_z}^* \]
- However, if the order is stated incorrectly, it indicates a misunderstanding of the energy levels of the orbitals.
- Conclusion: If the order is stated as σ2pz before π2p, this statement is incorrect.

### Final Answer
The statement that is not correct from the viewpoint of molecular orbital theory is **Option 4**.