An ionic compound is a compound composed of ions, which are atoms or molecules that have lost or gained electrons, resulting in a net electric charge. Ionic compounds are formed when the electronegativity difference between two elements is large enough that one element completely gives up its valence electrons to the other element.

Na and K are both in the same group of the periodic table and have very similar electronegativities. So, neither element is able to completely give up its valence electrons to the other, and the two elements do not form an ionic compound.