Calcium reactivity vs electronic configuration
The electronic configuration of Ca is $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^2}$ 

It has a total 4 Energy levels. The outermost energy level has 2 electrons called valance electrons. As, Calcium is metal and it can easy donate two electrons and becomes cation (Ca+2). Now, this cation is stable. 

Calcium is reactive because it can easy donate electrons by providing energy to it called ionisation energy.