Question

What periodic trend exist for ionization energy and how does this trend relate to different energy levels?

Answer 1

To repeat an old answer: ionization energy INCREASES across a PERIOD (left to right) but DECREASES down a Group.

Explanation:

This is one of the most important Periodic trends you can learn, in that it highlights the effects of periodicity. As `Z`, the atomic number increases sequentially, an electron is added to the same shell. In incomplete shells (i.e., the same Period), electrons shield each other very imperfectly from the the increased nuclear charge. Atomic radii thus decrease markedly across the Period from left to right: neon is a much smaller atom than lithium. Ionization energy would thus INCREASE markedly across the Period because of the increased effective nuclear charge.

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.