Ionic radius trends are seen within a group in the periodic table, where the radius gets bigger as one moves down a group and across a period, where the radius decreases among positive ions and increases among negative ions. The ionic radius is the radius of an atom's ion, and can be either larger or smaller than the radius of the neutral atom.
As one moves down a group on the periodic table, the ionic radius increases because additional electrons are being added to the atom. Within a period, the radius trend is dependent on whether the ion is positive or negative. When moving across a period among positive ions, the number of protons increases. This decreases the ionic radius because the protons pull the negatively-charged electrons closer to the nucleus. Among negative ions, additional layers of electrons are being added, which increases the radius.