Here's why:

* Alkaline Earth Metals: These metals belong to Group 2 of the periodic table. They have a +2 charge when they form ions.

* Oxides: Oxides are compounds formed with oxygen. Oxygen typically has an oxidation state of -2.

Example:

Let's take the example of calcium oxide (CaO):

* Calcium (Ca) has an oxidation state of +2.

* Oxygen (O) has an oxidation state of -2.

To maintain neutrality in the compound, the sum of the oxidation states must equal zero. Therefore, the formula CaO is correct.

This pattern holds true for all alkaline earth metal oxides (BeO, MgO, CaO, SrO, BaO, and RaO).