Which pair of elements from the same group shows the greatest increase in atomic radius?
Atomic radius increases down a group, with the largest jump often between early periods. In group 1, Li (period 2) to Na (period 3) shows a significant increase due to the addition of a new shell, compared to later pairs.
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Modern Periodic Law and Present Form - Long Form Periodic Table