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

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.