Well, as we all know, atoms prefer a full valence shell. That being said, as we go right in a period, we are increasing e- and we are also adding protons. As we add protons, the atom gets smaller, because the nuclear charge will be more powerful.
If you move down on the periodic table, ionization energy decreases because you are adding a full new energy level. More electrons=more repulsion.
However, Although there're more shielding as you add more electrons, the nuclear charge also increases as you go from left to right because of the increase number of protons.
Hope that helps!
