Final answer:
Atomic radii increase as you go down a column and decrease as you go across a row on the periodic table.
Step-by-step explanation:
As you go down a column of the periodic table, the atomic radii increase. This is because the valence electron shell is getting larger and there is a larger principal quantum number, so the valence shell lies physically farther away from the nucleus. On the other hand, as you go across a row on the periodic table from left to right, the trend is different. The increasing positive charge casts a tighter grip on the valence electrons, so the atomic radii decrease.