Which of the following oxides have rock salt structure with coordination number `6:6`?

Options

A. `BeO`
B. `MgO`
C. `CaO`
D. `SrO`

Answer

To determine which of the given oxides have a rock salt structure with a coordination number of 6:6, we will analyze the properties of each oxide, particularly focusing on their crystal structures.

### Step-by-Step Solution:

1. **Understand Rock Salt Structure**:
- The rock salt structure is characterized by a face-centered cubic (FCC) lattice. In this structure, each ion is surrounded by six oppositely charged ions, leading to a coordination number of 6:6.

2. **Identify the Oxides**:
- The oxides in question are Beryllium oxide (BeO), Magnesium oxide (MgO), Calcium oxide (CaO), and Strontium oxide (SrO).

3. **Analyze Beryllium Oxide (BeO)**:
- Beryllium oxide has a different structure. Beryllium is small in size and forms a hexagonal close-packed (HCP) structure rather than an FCC structure. The coordination number here is 4:4, not 6:6. Therefore, BeO does not have a rock salt structure.

4. **Analyze Magnesium Oxide (MgO)**:
- Magnesium oxide has a rock salt structure. It adopts an FCC lattice where Mg²⁺ ions and O²⁻ ions are arranged in a manner similar to NaCl, resulting in a coordination number of 6:6.

5. **Analyze Calcium Oxide (CaO)**:
- Calcium oxide also exhibits a rock salt structure. Like MgO, it has an FCC lattice with a coordination number of 6:6.

6. **Analyze Strontium Oxide (SrO)**:
- Strontium oxide similarly has a rock salt structure. It follows the same FCC arrangement as MgO and CaO, resulting in a coordination number of 6:6.

7. **Conclusion**:
- The oxides that exhibit a rock salt structure with a coordination number of 6:6 are Magnesium oxide (MgO), Calcium oxide (CaO), and Strontium oxide (SrO).

### Final Answer:
The oxides that have a rock salt structure with coordination number 6:6 are **MgO, CaO, and SrO**.