Which of the following pairs of elements is most likely to form an ionic compound?1. Nitrogen and sulfur? OR 2. Magnesium and Fluorine?

Magnesium and fluorine.

Explanation:

Ionic compounds are formed by the electrostatic attraction of cations and anions.

Cations, positive ions, are formed when atoms lose electrons, and anions, negative ions, are formed when atoms gain electrons.

When two different atoms have similar atraction for electrons (electronegativity) they will not donate to nor catch electrons from each other, so cations and anions will not be formed. Instead, the atoms would prefer to share electrons forming covalent bonds to complete their outermost shell (octet rule).

Then, in order to form ionic compounds the electronegativities have to substantially different. This situation does not happen between two nonmetal elements, which nitrogen and sulfur are. Then, you can predict safely that nitrogen and sulfur will not form an ionic compound.

Ionic compounds, then require the electronegativity difference that exist between some metals and nonmetals. Being magnesium an alkaline earth metal, its electronegativity is very low. On the other hand, fluorine the first element of the group 17, has the highest electronegativity of all the elements. Thus magnesium and fluorine will have enough electronegativity difference to justify the exchange of electrons, forming ions and, consequently, ionic compounds.