Which of the two is more hard `:MgO or CaO`? The internuclear distances of `MgO and CaO` are 2.05 and `2.40Å` respectively.

Explanation

To determine which of the two compounds, MgO (Magnesium Oxide) or CaO (Calcium Oxide), is harder, we can analyze the given internuclear distances and the ionic sizes of the elements involved.

### Step-by-Step Solution:

1. **Understand the Ionic Sizes**:
- Magnesium (Mg) is located above Calcium (Ca) in Group 2 of the periodic table. As you move down a group, the size of the atoms increases due to the addition of electron shells. Therefore, the ionic radius of Ca²⁺ is larger than that of Mg²⁺.

2. **Given Internuclear Distances**:
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