Which statements are correct for the peroxide ion ?
(1) It has five completely filled anti - bonding molecular orbitals
(2) It is diamagnetic
(3) It has bond order one
(4) It is isoelectronic with neon

Options

A. 1,2
B. 2,3
C. 2,4
D. 1,2,3

Answer

To determine which statements are correct for the peroxide ion (O2²⁻), we will analyze each statement based on molecular orbital theory and the properties of the peroxide ion.

### Step-by-Step Solution:

1. **Determine the Total Number of Electrons:**
The peroxide ion (O2²⁻) has 2 oxygen atoms, each contributing 8 electrons, plus 2 additional electrons due to the 2 negative charges.
\[ \text{Total electrons} = 8 + 8 + 2 = 18 \text{ electrons} \]

**Hint:** Count the electrons from each atom and add any extra electrons from the charge.

2. **Construct the Molecular Orbital Diagram:**
The molecular orbital (MO) energy levels for O2²⁻ are filled as follows:
- \(\sigma_{1s}^2\)
- \(\sigma^*_{1s}^2\)
- \(\sigma_{2s}^2\)
- \(\sigma^*_{2s}^2\)
- \(\sigma_{2p_z}^2\)
- \(\pi_{2p_x}^2\)
- \(\pi_{2p_y}^2\)
- \(\pi^*_{2p_x}^2\)
- \(\pi^*_{2p_y}^2\)
- \(\sigma^*_{2p_z}^0\)

After filling, we find:
- Bonding MOs: 10 electrons
- Antibonding MOs: 8 electrons

**Hint:** Use the order of filling molecular orbitals based on energy levels.

3. **Calculate the Bond Order:**
The bond order can be calculated using the formula:
\[ \text{Bond Order} = \frac{(\text{Number of bonding electrons} - \text{Number of antibonding electrons})}{2} \]
Substituting the values:
\[ \text{Bond Order} = \frac{(10 - 8)}{2} = 1 \]

**Hint:** Remember that bond order indicates the strength and stability of the bond.

4. **Check Each Statement:**
- **Statement 1:** "It has five completely filled anti-bonding molecular orbitals."
- **Analysis:** There are only 4 completely filled antibonding MOs. This statement is **incorrect**.

- **Statement 2:** "It is diamagnetic."
- **Analysis:** Since all electrons are paired, the peroxide ion is **diamagnetic**. This statement is **correct**.

- **Statement 3:** "It has bond order one."
- **Analysis:** We calculated the bond order to be 1. This statement is **correct**.

- **Statement 4:** "It is isoelectronic with neon."
- **Analysis:** Neon has 10 electrons, while the peroxide ion has 18 electrons. They are not isoelectronic. This statement is **incorrect**.

5. **Final Conclusion:**
The correct statements about the peroxide ion are:
- Statement 2: It is diamagnetic.
- Statement 3: It has bond order one.

### Summary of Correct Statements:
- The correct statements for the peroxide ion are **2 and 3**.