# Why does the ionization energy of the halogen atoms DECREASE down the Group?

Apr 23, 2017

Would you not expect this order of ionization energies?

#### Explanation:

Ionization energy interrogates the process:

$X \left(g\right) + \Delta \rightarrow {X}^{+} \left(g\right) + {e}^{-}$

Now ionization energies increase across a Period (from left to right as we face the Table), but decrease DOWN a Group. As a new valence shell is occupied down the Group, the more distant valence electrons should be LESS tightly held by the nuclear charge, even tho $Z$ has increased.

I can just read the diagram with glasses. I hope your eyes do better.