CBSE Class 10 Science 2017 · Paper 31/1/1/C · Question 12

(i) From left to right in a period, the atomic radius decreases (in period 2 it falls from lithium, $152$ pm, to oxygen, $66$ pm). Reason: the nuclear charge increases by one unit from each element to the next, while the added electron goes into the same outermost shell; the greater nuclear charge pulls the electrons closer to the nucleus and reduces the size of the atom. (ii) Down a group, the atomic radius increases (in group 1 it rises from lithium, $152$ pm, to caesium, $262$ pm). Reason: a new shell is added at each step down the group, which increases the distance between the outermost electrons and the nucleus, so the atomic size increases in spite of the increase in nuclear charge.