# Using ideas of electronic structure, what happens to the radii of the atoms in group 1 or 2, when the group is descended?

For groups 1 and 2 elements, the most loosely-bound electron will occupy an $s$-orbital ($1 s$ for period 1, $2 s$ for period 2, $3 s$ for period 3, and so on). The successive orbitals are larger in size, meaning the probability of an electron being farther from the nucleus gets greater as you go down the group.