The atomic radius of nonbonded atoms changes as you go across a row, or period, on the periodic table. It also changes as you go down a column on the periodic table. Elements with the smallest atomic radii are located on the upper right corner of the periodic table, while elements with the largest atomic radii are located on the lower left corner of the periodic table.
In general, as you move across a period (row), atomic size decreases because of increasing positive charge in the nucleus and increasing negative charge with added electrons. These charges attract each other. However, when a new energy level is added to the atom, as seen at the beginning of each row, the atomic size increases dramatically.