Atomic radius

The atomic radius of an element is the distance between the nucleus and the edge of the electron cloud. 
The atomic radius is rather hard to define, because it is impossible to know where all of the electrons are at a given time. 
In the periodic table of the elements, the atomic radius tends to increase as you move down a group because the number of electrons is greater, and so the radius of the circle of electrons in the element is bigger. As you move to the right across a period, the atomic radius tends to decrease due to the shielding effect.