What trends do you see on the periodic table for Ionization energy? What causes these trends?
Well, ionization energy..........INCREASES across a Period.......
Well, ionization energy INCREASES across a Period, an horizontal row of the Periodic Table from LEFT to RIGHT as we face the Table, and DECREASES down a Group, a column of the Periodic Table.
Two factors, influence this trend: (i) Nuclear Charge, and (ii) shielding by other electrons.
See here and here. Incomplete electronic shells shield the nuclear charge VERY ineffectively. And thus across the Period, the atomic radii markedly contract - this is a macroscopic variable that you should investigate. Once a valence shell is completed, shielding becomes reasonably effective, and electrons add to a valence shell farther removed from the nuclear core.