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

Options

A. `BaO`
B. `MgO`
C. `KCI`
D. `NaCI`

Explanation

To determine which of the given substances has the highest melting point, we can follow these steps:

### Step 1: Identify the substances and their ionic charges
We are given four substances, and we need to examine their ionic charges:
1. KCl: K⁺ (charge +1), Cl⁻ (charge -1)
2. NaCl: Na⁺ (charge +1), Cl⁻ (charge -1)
3. BeO: Be²⁺ (charge +2), O²⁻ (charge -2)
4. MgO: Mg²⁺ (charge +2), O²⁻ (charge -2)

### Step 2: Understand the relationship between ionic charge and melting point
The melting point of ionic compounds is influenced by the charges of the cations and anions. Higher charges lead to stronger electrostatic attractions between the ions, resulting in a higher melting point.

### Step 3: Compare the charges of the ions
- KCl and NaCl both have ions with charges of +1 and -1.
- BeO and MgO have ions with charges of +2 and -2.

Since BeO and MgO have higher charges compared to KCl and NaCl, they are likely to have higher melting points.

### Step 4: Consider the size of the ions
The melting point is also influenced by the size of the ions. When ions are of comparable size, they can pack more closely together, leading to increased lattice energy and thus a higher melting point.

- In the case of BeO, Be²⁺ is smaller than Mg²⁺, and O²⁻ is the same in both compounds. However, the size difference between Be²⁺ and O²⁻ is significant, which may affect the packing efficiency.
- MgO has ions that are more comparable in size, leading to better packing and higher lattice energy.

### Step 5: Conclusion
Based on the charges and the size of the ions, we conclude that:
- KCl and NaCl have lower melting points due to their +1 and -1 charges.
- BeO has a higher melting point than KCl and NaCl but is still lower than MgO due to the size difference.
- MgO has the highest melting point because of its +2 and -2 charges and the comparable size of its ions, which allows for better packing.

Thus, the substance with the highest melting point is **MgO**.