Which of the following statements is/are true for all the alkali metals ?

Options

A. Their nitrates decomose on heating to give `NO_(2) and O_(2)`
B. Their carbonates decompose on heating to give `CO_(2)` and metal oxide
C. They react with oxygen to give mainly the oxide `MO_(2)`
D. They react with halogens to give haildes `MX`

Answer

To determine which statements are true for all alkali metals, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Their nitrates decompose on heating to give NO2 and O2.**
- Alkali metal nitrates generally decompose upon heating to form metal nitrites and oxygen. However, lithium nitrate (LiNO3) is an exception as it decomposes to give lithium oxide (Li2O), nitrogen dioxide (NO2), and oxygen (O2). Therefore, this statement is **not true for all alkali metals**.

2. **Statement 2: Their carbonates decompose on heating to give CO2 and metal oxide.**
- Most alkali metal carbonates are thermally stable and do not decompose upon heating. The only exception is lithium carbonate (Li2CO3), which decomposes to form lithium oxide (Li2O) and carbon dioxide (CO2). Thus, this statement is also **not true for all alkali metals**.

3. **Statement 3: They react with oxygen to give mainly the oxide MO2.**
- Alkali metals can react with oxygen to form different types of oxides. For example, lithium forms lithium oxide (Li2O), sodium can form sodium oxide (Na2O) or sodium peroxide (Na2O2), and potassium, rubidium, and cesium can form superoxides (KO2, RbO2, CsO2). Since not all alkali metals form the oxide MO2, this statement is **not true for all alkali metals**.

4. **Statement 4: They react with halogens to give halides of type MX.**
- Alkali metals readily react with halogens to form halides of the type MX, where M is the alkali metal and X is the halogen. For example, sodium reacts with chlorine to form sodium chloride (NaCl). This behavior is consistent across all alkali metals, making this statement **true for all alkali metals**.

### Conclusion:
The only statement that is true for all alkali metals is **Statement 4**: They react with halogens to give halides of type MX.