The elements in the periodic table are arranged in order of increasing atomic numbers. The horizontal rows are called as periods and vertical columns are called as groups.
The elements in the groups are arranged in order of increasing atomic numbers and thus the electrons are being added.The electrons are added to new energy shells.
But the effect of addition of new shells outweighs the increase of nuclear charge and thus the atomic radius of the atoms will keep on increasing as we move down a column in the periodic table.
Tomic radius INCREASES as you go DOWN a Group because each successive Period (row) has an additional occupied energy level. you can think of it this way: each time you drop down a row, you add a "ring."
