Question

What element readily forms an ion which has the same electron arrangement as Krypton?

Answer 1

Well there are two elements that are more eager to make the Krypton configuration.

One is Bromine, where it wants to gain one more electron to form the Br- ion, which will have a Krypton configuration.

And the other is Rubidium, where it wants to lose one electron to form the Rb+ ion, which will also have the Krypton configuration.

I'm not 100% sure if this answers your question, but I hope it helps regardless :)

Answer 2

think about electronegativity :)