How does ionization energy change?
Ionization energies increase across a Period; they decrease down a Group.
Across the Period, from left to right, we add electrons to the same valence shell but sequentially increase
Down a Group, the valence electrons are further removed from the nucleus, and inner electronic shells effectively shield the nuclear charge. The result? Ionization energies decrease.
You should back this argument up with data to show that I am not talking complete bullocks. Here is a start.