As we move along a period from left to right in the periodic table, number of shells remains the same while nuclear charge increases by one at each step. Due to increase in nuclear charge electrons in the outermost shell are attracted with increasing force resulting in a gradual decrease in atomic size along a period. As we move down a group from top to broom, number of shells as well as nuclear charge increases. However, this effect of increase in the number of shells dominates over increase in nuclear charge. As such, atomic size increases down a group in the periodic table.
