Give the decreasing order of melting points of the following compounds:
i. NaF ii. BeO iii. MgO iv. SrO
Given: The inter-ionic distance in `Å` as
`NaF=2.31, BeO=1.65, MgO=2.106, SrO=2.58`
b. Given the decreasing order of hardness of the following compounds:
i. CaO ii. BeO iii. TiC
Given : The inter-ionic distances in `Å` as
`CaO=2.405, BaO=2.762, TiC=2.159`

Explanation

To solve the problem, we will analyze the melting points and hardness of the given compounds based on their lattice enthalpy, which is influenced by the charges of the ions and the inter-ionic distances.

### Part A: Decreasing Order of Melting Points

1. **Understanding Lattice Enthalpy**:
The melting point of ionic compounds is directly proportional to their lattice enthalpy. The formula for lattice enthalpy (U) is given by:
\[
U \propto \frac{Q_1 \times Q_2}{R_0}
...