The element which on burning in air gives proxide is

Options

A. lithium
B. sodium
C. rubidium
D. cesium

Explanation

To determine which element, when burned in air, produces a peroxide, we can analyze the behavior of the alkali metals (s-block elements) with oxygen. Let's go through the options step by step.

### Step-by-Step Solution:

1. **Understanding Peroxides**:
- Peroxides are compounds that contain an O-O bond. For example, sodium peroxide (Na2O2) is a well-known peroxide.

2. **Analyzing the Options**:
- We need to evaluate each of the given elements (let's assume they are lithium, sodium, rubidium, and cesium) to see which one forms a peroxide when burned in air.

3. **Option A: Lithium (Li)**:
- When lithium burns in oxygen, it forms lithium oxide (Li2O).
- Reaction: \( 4Li + O2 \rightarrow 2Li2O \)
- **Conclusion**: Lithium does not form a peroxide.

4. **Option B: Sodium (Na)**:
- Sodium reacts with oxygen to form sodium peroxide (Na2O2).
- Reaction: \( 2Na + O2 \rightarrow Na2O2 \)
- **Conclusion**: Sodium forms a peroxide.

5. **Option C: Rubidium (Rb)**:
- Rubidium reacts with oxygen to form rubidium superoxide (RbO2).
- Reaction: \( 4Rb + O2 \rightarrow 2Rb2O2 \) (but primarily forms superoxide)
- **Conclusion**: Rubidium does not form a peroxide but forms superoxide.

6. **Option D: Cesium (Cs)**:
- Cesium also reacts with oxygen to form cesium superoxide (CsO2).
- Reaction: \( 4Cs + O2 \rightarrow 2Cs2O2 \) (but primarily forms superoxide)
- **Conclusion**: Cesium does not form a peroxide but forms superoxide.

7. **Final Answer**:
- Based on the analysis, the element that burns in air to give a peroxide is **Sodium (Na)**.