Question

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

Answer 1

Would you not expect this order of ionization energies?

Explanation:

Ionization energy interrogates the process:

`X(g) + Delta rarr X^(+)(g) + 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.