Ionic bond is a chemical bond formed by the complete transfer of electrons between two atoms. The atom that loses electrons gains a positive charge (cation) and that which accepts electrons gains a negative charge (anion). Now, electronegativity is a parameter that measures the tendency of an atom to accept electrons. In the context of ionic bonding, two elements which show a significant difference in their electronegativity values form ionic bonds.
In the given examples, the difference in electronegativity is greatest between K and Br i.e. 0.8 and 2.8 respectively with a difference of 2.0. This also makes sense since K and Br are on the extreme ends of the periodic table. Hence, potassium with a valence electron configuration of 4s1 will lose its s electron to Br (4s24p6) and form an ionic molecule K⁺Br⁻
