Which of the following substance has the highest melting point? .

Options

A. `NaCl`
B. `KCl`
C. `MgO`
D. `BaO`

Answer

To determine which of the given substances has the highest melting point among sodium chloride (NaCl), potassium chloride (KCl), magnesium oxide (MgO), and barium oxide (BaO), we can analyze their melting points based on their lattice energies and the charges of their ions.

### Step-by-Step Solution:

1. **Understanding Melting Point and Lattice Energy**:
- The melting point of ionic compounds is influenced by their lattice energy, which is the energy required to separate one mole of an ionic solid into its gaseous ions.
- Lattice energy is directly proportional to the product of the charges of the ions (Q1 and Q2) and inversely proportional to the distance (R) between the ions:
\[ \text{Lattice Energy} \propto \frac{Q1 \times Q2}{R} \]

2. **Identifying Charges on Ions**:
- For sodium chloride (NaCl): Na⁺ (charge +1) and Cl⁻ (charge -1).
- For potassium chloride (KCl): K⁺ (charge +1) and Cl⁻ (charge -1).
- For magnesium oxide (MgO): Mg²⁺ (charge +2) and O²⁻ (charge -2).
- For barium oxide (BaO): Ba²⁺ (charge +2) and O²⁻ (charge -2).

3. **Comparing Charges**:
- NaCl and KCl have lower charges (+1 and -1).
- MgO and BaO have higher charges (+2 and -2).
- Higher charges generally lead to higher lattice energies and thus higher melting points.

4. **Considering Ionic Radii**:
- The distance (R) between the ions also affects lattice energy. As you move down a group in the periodic table, ionic radii increase.
- Therefore, BaO will have a larger distance between ions compared to MgO, which will decrease its lattice energy.

5. **Conclusion**:
- Although both MgO and BaO have the same charges, MgO has a smaller ionic radius leading to a smaller distance (R) between the ions.
- Thus, MgO will have the highest lattice energy and consequently the highest melting point among the given substances.

### Final Answer:
The substance with the highest melting point is **Magnesium Oxide (MgO)**.

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