Ihave 2 questions:
sulfur has an atomic number of 16 and belongs in group 6 on the periodic table. calcium has an atomic number of 20 and belongs in group 2 on the periodic table. which element has a greater reactivity and why?
how do you know if an element in reactive or not?

The reactivity depends on the elements valence electrons. The need to be stable will make an element become more reactive, In order for an atom to be stable, it needs 8 valence electrons in its outer shell. 

Since Calcium is in group 2, it has 2 valence electrons, and would rather give up the electrons. While Sulfur has 6, it would need to gain 2 more to be stable, making it more reactive. 

An atoms valence is the amount of electrons in the outer ring and atoms like to have a valence of 8 so the closer to having 8 in the outer ring the more reactive the atom is so since both elements are 2 away from a valence of 8 theyre equally reactive