Ca + O2

The reaction between calcium (Ca) and oxygen (O2) produces calcium oxide (CaO). This is a combination reaction where the elements react to form a compound. The balanced chemical equation for this reaction is:

\[ 2 \, \text{Ca} + \text{O}_2 \rightarrow 2 \, \text{CaO} \]

In this equation, two moles of calcium react with one mole of oxygen to produce two moles of calcium oxide. Calcium oxide is an ionic compound, and it is commonly known as lime. It is caustic and can react with water to form calcium hydroxide (Ca(OH)₂), also known as slaked lime.