What is ionic radius?

Hint:The ionic radius of an ion is the effective distance between the nucleus and the outermost shell of an ion having an electron. In other words we can say that the ionic radius is the distance from the nucleus up to which it has influence on its electron cloud.

Complete step-by-step answer:Let us learn more about the ionic radius in detail;
Ionic radius-
Generally, as we all know, ions are formed when either an atom loses or gains electrons. When an atom loses an electron, cation is formed whose ionic radius is less than that of the parent atom. When an atom gains an electron, anion is formed which is comparatively larger in size than the parent atom.
Trends in the modern periodic table-
While moving down the group, ionic radius increases. Whereas, across the period, atoms show haphazard behaviour with respect to its ionic size. If considering period 3, the ionic radius will first decrease and then suddenly increase and then again slowly decreases.

Note:Do note that the atomic radius differs from the ionic radius. The atomic radius measures the normal size of the neutral atom whereas, ionic radius measures the size of the electrically charged atom.