# Why does atomic size increase down a group?

Feb 23, 2016

On the other hand, going down a Group, we go to another so-called shell of electrons, that build on the preceding shell. Atomic radii thus INCREASE down the Group. This contest between nuclear charge, i.e. $Z$, and shielding by other electrons, underlies the structure of the Periodic Table. And note that incomplete valence electronic shells, shield the nuclear charge VERY ineffectively.
AS a scientist, however, you should seek data that inform your argument. And I leave this up to you. You want details of $\text{atomic radii}$ only, not $\text{ionic radii}$! If you can remember $\text{Atomic size INCREASES down a Group, but DECREASES}$ $\text{across a Period,}$ where a Group is a column and Period is a row of the Periodic Table, you have mastered a fundamental principle of chemistry.