Take a look at the periodic table shown in the attached picture. Group 17 elements are enclosed in a red box as shown. Now, let us find the electronegativities of those elements.
F: 3.98
Cl: 3.16
Br: 2.96
I: 2.66
At: 2.2
Find the difference of the electronegativities. The one with the greatest difference is the answer.
With F: 3.98-0.8 = 3.18
With Cl: 3.16-0.8 = 2.38
With Br: 2.96-0.8 = 2.16
With I: 2.66-0.8 = 1.88
With At: 2.2-0.8 = 1.4
Thus, the answer is with F. The compound would be KF.
