Question

The following is in order of increasing ionization energy:

As, Br, Kr, Rb+

Can someone please explain to me why Rb+ has higher ionization energy when i thought everything moving to the top right of the periodic table had the highest ionization energy?

Answer 1

Increasing order of ionisation energy is :- As < Br < Kr < Rb^+

From left to right ionisation energy increases, therefore it is expected that As < Br < Kr are in increasing order of ionisation energy.

From top to bottom ionisation energy decreases. Therefore Rb has less ionisation energy than As, Br and Kr.

But Rb^+ has higher ionisation energy than As, Br and Kr. This is because Rb^+ is positively charged ion. And energy to remove electron from positive charged ion is always higher than from neutral atom.

This is due to fact as nuclear charge increases, it's attraction for outermost electron increases and more energy is required to remove valance electron.

Therefore as Rb^+ has high nuclear charge it's ionisation energy is higher even though it is in bottom of periodic table.