Final answer:
The atomic radius increases from top to bottom within a group in the periodic table, due to an increase in the number of occupied principal energy levels that outweighs the increase in nuclear charge. Conversely, it decreases from left to right across a period because of the increasing pull of the nucleus on the same energy level valence electrons.
Step-by-step explanation:
Periodic Trend of Atomic Radii
When considering the trend of atomic radii within the periodic table, it's important to note the clear periodic patterns that emerge. As you move from top to bottom within a group (a column), the atomic radius increases. This happens because as the atomic number (Z) increases, the number of occupied principal energy levels grows, resulting in orbitals that are farther away from the nucleus and larger in size. The increase in positive nuclear charge is outweighed by the greater number of principal energy levels, hence the increase in atomic radius.
Conversely, from left to right across a period (a row), the atomic radius decreases as Z increases. This is because while the valence electrons are added in the same energy level, there is a progressive increase in the nuclear charge due to the addition of more protons to the nucleus, which pulls the valence electrons closer, making the atom smaller.