Question

Which of the following pairs of atoms exhibit an ionic bond?


H and S

Fe and O

C and F

C and N

Answer 1

Fe & O makes an ionic bond because of this 
Metal and Metal = Metallic
Non-Metal and Metal = Ionic
Non-Metal and Non-Metal = Covalent
In this case Oxygen (O) is a Non-Metal and Iron (Fe) is a Metal so you have an Ionic Bond

Answer 2

Fe and O because Fe (Iron) is a metal, and O(oxygen) is a nonmetal. Ionic bond forms between a metal and a nonmetal, not nonmetal and nonmetals like C and N