Rank the following from smallest first ionization energy to largest first ionization energy: As3−KrRb+Se2−

Answer 1

The smallest ionization energy will be that of As3-, followed by Se2-, Kr and Rb+.

The reason why is because all of these species have equal number of electrons, but a different number of protons. Since As3- has lowest proton:electron ratio, it is going to be easiest to take an electron away from it, while it is going to be very hard to take it from Rb+.