Final answer:
Ionization energy increases across a period and decreases down a group.
Step-by-step explanation:
The ionization energy of elements increases across a period and decreases down a group. Across a period, ionization energy increases because the number of valence electrons increases and the electrons are pulled in tighter to the nucleus. Down a group, ionization energy decreases because the valence electrons are further away from the nucleus and experience less pull.