What trend in atomic radius do you see as you go across a period/row on the periodic table. What causes this trend?
I think you clearly know that atomic radii DECREASE across a Period reading from left to right. Why??
Two factors are in opposition: (i) nuclear charge; and (ii) shielding by other electrons. Since nuclear charge
Thus, when electrons are added to the same valence shell, they tend to shield each other (i.e. by electron-electron repulsion) VERY imperfectly. So the effect of nuclear charge wins, and atomic radii decrease.
On the other hand, once the valence shell is full, electronic shielding of the nuclear core becomes more effective. The next valence shell is further removed from the nucleus, and the whole shooting match of shielding versus nuclear charge begins again.