Answer: Ionic radii is half the distance between two ions.
Explanation: from top to bottom down an element group, ionic radius increases. This is because a new electron shell is added as you move down the periodic table.
From left to right across an element period the ionic radius decreases. Even though the size of the atomic nucleus increases with larger atomic numbers moving across a period, the ionic and atomic radius decreases.
The above stated trends are the same for metals and non metals in the periodic table.