The alkaline earth metal nitrate that does not crystallise with water molecules, is :

Options

A. 1.`Mg(NO_(3))_(2)`
B. 2.`Sr(NO_(3))_(2)`
C. 3.`Ba(NO_(3))_(2)`
D. 4.`Ca(NO_(3))_(2)`

Explanation

To solve the question regarding which alkaline earth metal nitrate does not crystallize with water molecules, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Alkaline Earth Metals**:
The alkaline earth metals are located in Group 2 of the periodic table. These include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra).

2. **Understand Water of Crystallization**:
Water of crystallization refers to water molecules that are incorporated into the crystalline structure of a compound. The amount of water of crystallization can vary among different compounds.

3. **Analyze the Trend**:
As we move down the group from Beryllium to Barium, the size of the metal cation increases. This increase in size affects the ability of the metal to coordinate with water molecules.

4. **Relate Size of Cation to Water of Crystallization**:
Generally, smaller cations can accommodate more water molecules due to their higher charge density. Conversely, larger cations have a lower charge density and can hold fewer water molecules.

5. **Evaluate Each Nitrate**:
- Beryllium Nitrate (Be(NO3)2) typically crystallizes with 4 water molecules.
- Magnesium Nitrate (Mg(NO3)2) can crystallize with 6 water molecules.
- Calcium Nitrate (Ca(NO3)2) can crystallize with 4 water molecules.
- Strontium Nitrate (Sr(NO3)2) can crystallize with 3 water molecules.
- Barium Nitrate (Ba(NO3)2) does not crystallize with water molecules.

6. **Conclusion**:
Based on the analysis, Barium Nitrate (Ba(NO3)2) is the alkaline earth metal nitrate that does not crystallize with water molecules.

### Final Answer:
The alkaline earth metal nitrate that does not crystallize with water molecules is **Barium Nitrate (Ba(NO3)2)**.

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