Which of the following property of alkali metals/alkali metal salt increase with increase in atomic number ?

Options

A. Solubility of their hydroxides
B. Thermal stability of their carbonates
C. Their softness
D. Their hydration energies

Explanation

To solve the question regarding which properties of alkali metals or alkali metal salts increase with an increase in atomic number, we will analyze each property mentioned in the question step by step.

### Step 1: Analyze the Solubility of Hydroxides
- **Observation**: The solubility of alkali metal hydroxides increases with an increase in atomic number.
- **Reasoning**: This occurs because as the atomic number increases, the lattice energy of the hydroxides decreases. Lower lattice energy means that the hydroxides are more soluble in water.

### Step 2: Analyze the Thermal Stability of Carbonates
- **Observation**: The thermal stability of alkali metal carbonates increases with an increase in atomic number.
- **Reasoning**: As the atomic number increases, the charge density of the cation decreases. A lower charge density leads to less distortion of the carbonate ion, resulting in increased thermal stability.

### Step 3: Analyze the Softness of Alkali Metals
- **Observation**: The softness of alkali metals increases with an increase in atomic number.
- **Reasoning**: The increase in atomic number leads to an increase in the size of the metal atoms. Larger atoms have electrons that are more loosely held, making them softer. Additionally, as the atomic number increases, the lattice energy decreases, contributing to increased softness.

### Step 4: Analyze the Hydration Energy
- **Observation**: The hydration energy of alkali metal ions decreases with an increase in atomic number.
- **Reasoning**: As the atomic number increases, the size of the cation increases. Larger cations have a lower charge density, which results in a decrease in hydration energy. Thus, this property does not increase with atomic number.

### Conclusion
Based on the analysis:
- The properties that increase with an increase in atomic number are:
- **A**: Solubility of hydroxides
- **B**: Thermal stability of carbonates
- **C**: Softness of alkali metals

The correct answer is **A, B, and C**.