The balanced equation for the reaction of calcium (Ca) and oxygen gas (O₂) is:

2 Ca + O₂ → 2 CaO

Here's a breakdown:

* 2 Ca: Two atoms of calcium are needed to react.

* O₂: One molecule of oxygen gas (containing two oxygen atoms) is required.

* 2 CaO: The reaction produces two molecules of calcium oxide (CaO).

This equation is balanced because there are equal numbers of each type of atom on both sides of the equation:

* Calcium (Ca): 2 on each side

* Oxygen (O): 2 on each side