Arrange the followin in the decreasing order of the property mentioned :
`(i) Li^(+), Na^(+), K^(+), Rb^(+) `(Ionic mobility )
(ii) `Be , Mg , Ca , Sr ` ( Melting point )
(iii) BeO , MgO , CaO (Enthalpy of formation )
(iv) Be , Mg , Ca ( Metallic radius )

Explanation

Let's solve the question step by step for each part mentioned.

### Part (i): Ionic Mobility of Li⁺, Na⁺, K⁺, Rb⁺

1. **Understand Ionic Mobility**: Ionic mobility refers to how easily an ion can move through a medium, typically a solvent like water. Smaller ions with higher hydration energy tend to have lower mobility because they attract more water molecules around them.

2. **Evaluate Hydration Energies**:
- Li⁺ has the highest hydration energy due to its small size, which means it is surrounded by more water molecules, making it less mobile.
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