Among the following which metal forms superoxides of type `MO_2` ?

Options

A. K
B. Rb
C. Cs
D. All of these

Answer

To determine which metal forms superoxides of the type \( MO_2 \), we will analyze the properties of the given alkali metals: potassium (K), rubidium (Rb), and cesium (Cs).

### Step-by-Step Solution:

1. **Identify the Alkali Metals**: The metals in question are potassium (K), rubidium (Rb), and cesium (Cs). These are all alkali metals.

2. **Understand Superoxides**: Superoxides are compounds that contain the superoxide ion \( O_2^- \). The general formula for superoxides is \( MO_2 \), where M is the metal.

3. **Review Alkali Metal Oxides**:
- Lithium (Li) forms lithium oxide \( Li_2O \).
- Sodium (Na) forms sodium peroxide \( Na_2O_2 \).
- Potassium (K), rubidium (Rb), and cesium (Cs) can form superoxides \( KO_2 \), \( RbO_2 \), and \( CsO_2 \) respectively.

4. **Size of Cations**: The size of the alkali metal cations increases down the group:
- Lithium (Li) is the smallest.
- Sodium (Na) is larger than lithium.
- Potassium (K) is larger than sodium.
- Rubidium (Rb) is larger than potassium.
- Cesium (Cs) is the largest.

5. **Formation of Superoxides**:
- The larger cations (K, Rb, Cs) have a weaker positive field, which allows them to stabilize the larger superoxide ion \( O_2^- \).
- The lattice energy also favors the formation of superoxides for these larger cations.

6. **Conclusion**: Since potassium, rubidium, and cesium can all form superoxides, the answer to the question is that all three metals (K, Rb, Cs) form superoxides of the type \( MO_2 \).

### Final Answer:
**All of the above (potassium, rubidium, and cesium) form superoxides of the type \( MO_2 \).**