Periodic trends and effective nuclear charge

[henry3369]

My book is trying to explain why atomic radii decreases as you move toward the right side of the periodic table because the effective nuclear charge increases. I understand why an increase in effective nuclear charge results in a smaller radius, but I don't know why the effective nuclear charge would decrease with more valence electrons. If Z_eff= Z-S, then an increase in valence electrons would increase shielding; thus, Z_eff would be smaller with an increase in S.

 

[mfb]

You increase the proton number Z by 1, but the new electron won't be completely inside so its shielding effect is less than one elementary charge.