Trends in first ionization energies

[cristinapadilla_1E]

What are the trends in first ionization energies throughout the periodic table?

[Aaron Ang 4H]

First Ionization energies typically decrease down a group and increase across a period (from left to right)

[annabel 2A]

Across a period (from left to right), first ionization energy increases. This makes sense since across a period, the number of protons are increasing while the valence electrons are being added to the same energy shell (so they aren't further from the nucleus). This means the nucleus can hold onto the electrons more tightly as the nucleus gets more positive across the period.
Down a group, first ionization energy decreases. This makes sense because down a group, the energy shells are increasing, so the valence electrons are further from the nucleus, and the nucleus cannot hold onto the valence electrons as tightly.

[Catherine Kim 3K]

First ionization energies increase from left to right on the period table. Electron shielding affects ionization energy.