ICSE Class 10 Answered

In a group, the size of an atom increases as one proceeds from top to bottom. This is due to the successive addition of shells as we move from one period to the next in a group.

In Group I size of hydrogen is smallest and that of caesium is the largest.

                    H   <    Li   <   Na   <     K <     Rb   <     Cs

                37pm   152pm 186pm   231pm   244pm   262pm

Ionization potential 

Ionization energy decreases with an increase in the atomic size, i.e., it decreases as one moves down a group.