Here's why:

* Calcium (Ca) is an alkaline earth metal in group 2 of the periodic table. It has a +2 charge.

* Sulfur (S) is a nonmetal in group 16 of the periodic table. It has a -2 charge.

* To form a neutral compound, the charges must balance. Since the charges are equal and opposite, the ratio of calcium to sulfur is 1:1.