CBSE Class 10 Science 2020 · Paper 31/1/1 · Question 26(b)

Atomic size is the distance between the centre of the nucleus and the outermost shell of an isolated atom. It is measured in picometres (pm), where $1\text{pm}=10^{-12}$ m. In a group: the atomic radius increases from top to bottom, because a new shell is added at each step down, which increases the distance between the outermost electrons and the nucleus in spite of the increase in nuclear charge. In a period: the atomic radius decreases from left to right, because the nuclear charge increases while the electrons are added to the same shell, so the nucleus pulls the outermost electrons closer.