What two atoms form an ionic bond

Elements on opposite sides of the periodic table

Explanation:

to form an ionic bond the elements have to have enough valence electrons to finish filling up the shell. Sodium Chloride for example, sodium has one valence electron and chlorine has 7 valence, so they form a bond together because their total valence electrons is 8 and finishes filling up the shell. You can visualize this with Lewis dot diagrams. Hope that helps!