Final answer:
The ionic radius decreases across a period and increases down a group on the periodic table.
Step-by-step explanation:
The ionic radius generally decreases across a period and increases down a group on the periodic table.
Across a period, as you move from left to right, the atomic number increases and the effective nuclear charge experienced by the electrons increases. This leads to a stronger attraction between the electrons and the nucleus, causing the atomic radius to decrease.
Down a group, as you move from top to bottom, the number of principal energy levels increases. The effect of the greater number of energy levels outweighs the increase in nuclear charge, resulting in an increase in the atomic radius.