Consider the following reaction `M+O_2 rarrMO_2` (M= alkali metal) (stable superoxide)

Options

A. M can not be Li and Na
B. M can not be Cs and RB
C. M can not be Li and Rb
D. None of these

Answer

To solve the question regarding the formation of stable superoxides by alkali metals in the reaction \( M + O_2 \rightarrow MO_2 \), we will analyze the behavior of different alkali metals in this context.

### Step-by-Step Solution:

1. **Identify the Alkali Metals**:
Alkali metals are found in Group 1 of the periodic table, which includes Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs).

**Hint**: Remember that alkali metals have one electron in their outermost shell, which influences their reactivity and the types of compounds they form.

2. **Understanding Oxide Formation**:
When alkali metals react with oxygen, they can form different types of oxides depending on their size and charge density:
- **Lithium** typically forms \( Li_2O \) (oxide).
- **Sodium** typically forms \( Na_2O_2 \) (peroxide).
- **Potassium, Rubidium, and Cesium** can form superoxides (\( KO_2, RbO_2, CsO_2 \)).

**Hint**: The type of oxide formed is influenced by the size of the metal cation and the stability of the resulting anion.

3. **Stability of Superoxides**:
The stability of superoxides increases down the group. This is because larger cations (like K, Rb, and Cs) can stabilize the larger superoxide anion (\( O_2^{2-} \)) due to their lower charge density compared to smaller cations (like Li and Na).

**Hint**: Larger cations have a lower charge density, which allows them to stabilize larger anions more effectively.

4. **Exclusion of Lithium and Sodium**:
- Lithium forms only oxides due to its small size and high charge density.
- Sodium tends to form peroxides rather than superoxides because it cannot effectively stabilize the larger superoxide anion.

**Hint**: Consider the trend in charge density and how it affects the types of oxides formed by different alkali metals.

5. **Conclusion**:
From the analysis, we conclude that:
- The alkali metals that can form stable superoxides are Potassium (K), Rubidium (Rb), and Cesium (Cs).
- Therefore, the metal \( M \) in the reaction \( M + O_2 \rightarrow MO_2 \) cannot be Lithium or Sodium.

**Final Answer**: The correct alkali metals that can form stable superoxides are Potassium, Rubidium, and Cesium.