Final answer:
The atomic radius is the size of an atom that increases when going down a column in the periodic table, due to an increase in the principal quantum number and electron shielding factors.
Step-by-step explanation:
The characteristic size of an atom that increases going down a column in the periodic table is known as the atomic radius. This trend occurs due to the increasing distance of the valence electron shell from the nucleus as you go down a group, with a corresponding increase in the principal quantum number. Additionally, the increase in electron shielding results in a reduced effective nuclear charge on the outermost electrons, making them less tightly bound to the nucleus and thus further away.
Periodic trends also show that the atomic radius decreases from left to right across a period because electrons are added to the same energy level while the number of protons in the nucleus increases, pulling the electrons closer due to a stronger nuclear charge.