What periodic trend exist for ionization energy and how does this trend relate to different energy levels?
To repeat an old answer: ionization energy INCREASES across a PERIOD (left to right) but DECREASES down a Group.
This is one of the most important Periodic trends you can learn, in that it highlights the effects of periodicity. As
On the other hand, once the valence shell is complete, shielding of the increased nuclear charge becomes effective. The next Period builds on a complete valence shell, and the atomic radius expands. Given that the valence electrons are further removed from the positive nucleus, ionization energies should decrease, and indeed they do. This interplay between shielding and nuclear charge underlies atomic structure. I urge you to look at actual data that list atomic radii, and ionization enthalpies to inform your argument.