In between the metals A and B both form oxide but B also forms nitride, when both burn in air so A and B are

Options

A. `Cs,K`
B. `Mg,Ca`
C. `Li,Na`
D. `K,Mg`

Answer

To solve the question, we need to identify the metals A and B based on the information given about their ability to form oxides and nitrides when they burn in air.

### Step-by-Step Solution:

1. **Identify the Group of Metals**:
- Both metals A and B are S-block elements. S-block elements include alkali metals (Group 1) and alkaline earth metals (Group 2).

2. **Analyze Metal A**:
- Metal A can form an oxide. All alkali metals and alkaline earth metals can form stable oxides.

3. **Analyze Metal B**:
- Metal B forms both an oxide and a nitride. Among the alkali metals, only lithium (Li) can form a nitride. Therefore, if B is an alkali metal, it must be lithium.

4. **Consider Other Metals**:
- For alkaline earth metals, magnesium (Mg) is known to form both oxides and nitrides. Magnesium reacts with oxygen to form magnesium oxide (MgO) and with nitrogen to form magnesium nitride (Mg3N2).

5. **Eliminate Other Possibilities**:
- Sodium (Na) and potassium (K) do not form nitrides, so they cannot be metal B. Therefore, they cannot be the answer.

6. **Conclusion**:
- Since metal A must be an alkali metal that forms an oxide (likely potassium, K, which forms K2O) and metal B must be magnesium (Mg), which forms both MgO and Mg3N2, we conclude:
- Metal A = Potassium (K)
- Metal B = Magnesium (Mg)

Thus, the answer is:
**A = Potassium (K), B = Magnesium (Mg)**.

### Final Answer:
**Option D: A is Potassium and B is Magnesium.**

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