Answer
a) Chlorine (Cl)
b) Strontium ion ($Sr^{2+}$)
c) Sulfur ion ($S^{2-}$)
Work Step by Step
a) Group 7A elements usually form ion with -1 charge and period 3 element which also belongs to group 7A is chlorine (Cl).
b) Among group 2A elements, Strontium (Sr) has the atomic number of 38. hence there is 38 no. of protons and 38 electrons in neutral Sr atom. So, $Sr^{2+}$ is that ion from group 2A with 36 electrons.
c) Among group 6A elements, sulfur has the atomic number of 16. Hence, there is 16 no. of protons and 16 electrons present in neutral S atom. So, $S^{2-}$ will have 18 electrons.
