# Comparision of the Ionisation Energy Change of the alkali metals and noble gases?

Currently I compare the ionisation energys of the alkali metals:

Li: $\pu{0.52 eV}$
Na: $\pu{0.50 eV}$
K: $\pu{0.42 eV}$

Where only a slight drop (appr. remain constant) occurs.

This ionisation energys occur in comparision at the noble gases:

Ne: $\pu{2.08 eV}$
Ar: $\pu{1.52 eV}$
Kr: $\pu{1.35 eV}$

Here we can observe a faster drop in the ionisation energy.

This diagram visualizes the numbers:

How can we explain that? My first attempt was for the alkali metals: Due to the rising effective nuclear charge (values of Clementi: Li 1.2, Na 2.5, K: 3.5) the orbital energys of the "highest" s-Orbital raughly remain constant (despite of rising quantum number n) and so the ionisation energy does (Koopmanns-Rule).

But the same explanation can be taken for the noble gases ($Z_\mathrm{eff}$ for highest s orbital: Ne 5.7, Ar 7,7, Kr 11.3) Here we have even a faster rise of the effective nuclear charge … but an immense drop of the ionisation energy!

So something is missing in my explanation. Does the energy of the s orbitals influenced by another important factor? Does somebody have a better explaination?

Data from Huheey: "Inorganic chemistry"

• chemistry.stackexchange.com/questions/66492/… – Nilay Ghosh Nov 12 '17 at 14:52
• @Nilay Ghosh As far as I can see relates this linked question Not much with mine. My question deals with the enormous change of the IE of the noble gases (relative to the Change of the alkali metals). – Nilsfrank99 Nov 12 '17 at 15:29