Does ionization energy change across a period?
Across a period, the ionization energy increases.
• From L to R, no new energy levels are added. The e- are closer to the nucleus and are strongly held by it. This makes it difficult to remove them.
• From L to R, the nuclear shielding also stays the same (i.e. there is the same number of filled inner e- shells protecting outer e- from the pull of the + nucleus).
• However, the nuclear charge (i.e. the # of p+ in the nucleus) increases. More protons are added, which pulls the e- closer.
As such, the energy required to remove an e- from the outermost shell of the atom increases.