Peroxide ion is present in :

Options

A. `MgO`
B. `CaO`
C. `Li_(2)O`
D. `BaO_(2)`

Answer

To determine where the peroxide ion (O2^2-) is present among the given compounds (MgO, CaO, Li2O, and BaO2), we need to analyze the oxidation states of oxygen in each compound.

### Step 1: Understanding the Peroxide Ion
The peroxide ion consists of two oxygen atoms bonded together with a -2 charge (O2^2-). In this ion, each oxygen has an oxidation state of -1.

### Step 2: Analyzing Each Compound

1. **MgO (Magnesium Oxide)**
- Magnesium (Mg) has an oxidation state of +2.
- Let the oxidation state of oxygen be X.
- The equation for charge balance is:
\[ +2 + X = 0 \implies X = -2 \]
- Since the oxidation state of oxygen is -2, MgO does not contain the peroxide ion.

2. **CaO (Calcium Oxide)**
- Calcium (Ca) has an oxidation state of +2.
- Let the oxidation state of oxygen be Y.
- The equation for charge balance is:
\[ +2 + Y = 0 \implies Y = -2 \]
- Since the oxidation state of oxygen is -2, CaO does not contain the peroxide ion.

3. **Li2O (Lithium Oxide)**
- Lithium (Li) has an oxidation state of +1.
- For two lithium atoms, the total positive charge is +2.
- Let the oxidation state of oxygen be Z.
- The equation for charge balance is:
\[ +2 + Z = 0 \implies Z = -2 \]
- Since the oxidation state of oxygen is -2, Li2O does not contain the peroxide ion.

4. **BaO2 (Barium Peroxide)**
- Barium (Ba) has an oxidation state of +2.
- There are two oxygen atoms, so let the oxidation state of each oxygen be W.
- The equation for charge balance is:
\[ +2 + 2W = 0 \implies 2W = -2 \implies W = -1 \]
- Since the oxidation state of oxygen is -1 for each oxygen atom, BaO2 contains the peroxide ion.

### Conclusion
The peroxide ion is present in **BaO2 (Barium Peroxide)**.

### Final Answer
**BaO2**

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