How does atomic size change down the alkali-metal group?

The atomic size of alkali metals increases as you move down the group from lithium to francium.1

This trend occurs because, with each successive period, an additional electron shell is added to the atom. Essentially, electrons are placed into a new shell outside the existing closed-shell core, which increases the overall radius of the atom like the layers of an onion.1

This increase in size is reflected in the measured atomic radii of the group. For example, the atomic radius increases from 1.67 Å for lithium to 2.98 Å for cesium. Because of this larger atomic size and the presence of only one weakly bound valence electron, alkali metals exhibit low density and loose crystallographic arrangements.2