How does the atomic radius change as we go :
(i) from left to right across a period
(ii) down a group in the periodic table ?

Explanation

On going :
(i) From left to right across a period, atomic radius decreases as one proton and one electron are added continuously, therefore effective nuclear charge increases.
(ii) Down a group in the periodic table, atomic radius increases because number of shells goes on increasing, therefore distance between nucleus and valence shell increases.