How would you explain how period and group trends in atomic radii are related to electron configuration?
Atomic radii are influenced by two factors: (i) nuclear charge; (ii) shielding by other electrons. We can address the Periodic trend.
Atomic radii DECREASE across a Period, but increase down a Group. Why?
As we add another proton to the nucleus we add another electron to the valence shell. As we go across the Period from left to right, NUCLEAR charge is dominant. Thus as we go across the Period the increased nuclear charge results in a contraction of the atomic nucleus. And we should check the trend in atomic nuclei; there is such a contraction in atomic radii.
On the other hand, as we descend a Group (a column), the filled electron shells effectively shield the nuclear charge. This is why down a Group, the atomic radii INCREASE.
This is a very important issue, and you should become very confident in explaining and rationalizing Periodic trends.