Peroxide ion is present in :

Options

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

Answer

To determine where the peroxide ion is present, we need to analyze the oxidation states of oxygen in the given compounds. The peroxide ion (O2^2-) consists of two oxygen atoms, each with an oxidation state of -1.

Let's analyze each option step by step:

### Step 1: Analyze MgO
- Magnesium (Mg) is an alkaline earth metal with an oxidation state of +2.
- The compound is neutral, so we can set up the equation:
\[ \text{Mg} + \text{O} = 0 \implies +2 + x = 0 \implies x = -2 \]
- Here, oxygen has an oxidation state of -2, indicating that it is an oxide ion (not a peroxide).

### Step 2: Analyze CaO
- Calcium (Ca) is also an alkaline earth metal with an oxidation state of +2.
- Setting up the equation:
\[ \text{Ca} + \text{O} = 0 \implies +2 + x = 0 \implies x = -2 \]
- Again, oxygen has an oxidation state of -2, indicating that it is an oxide ion (not a peroxide).

### Step 3: Analyze Li2O
- Lithium (Li) is an alkali metal with an oxidation state of +1.
- In Li2O, we have:
\[ 2(\text{Li}) + \text{O} = 0 \implies 2(+1) + x = 0 \implies x = -2 \]
- Here, oxygen also has an oxidation state of -2, indicating that it is an oxide ion (not a peroxide).

### Step 4: Analyze BaO2
- Barium (Ba) is an alkaline earth metal with an oxidation state of +2.
- In BaO2, we have:
\[ \text{Ba} + 2(\text{O}) = 0 \implies +2 + 2x = 0 \implies 2x = -2 \implies x = -1 \]
- Here, each oxygen has an oxidation state of -1, indicating that it is a peroxide ion (O2^2-).

### Conclusion
From the analysis, we find that the peroxide ion is present in **BaO2**.

### Final Answer
The peroxide ion is present in: **BaO2**.

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