The atomic radii of the alkali metals increase down the group. This is because the number of energy levels increases down the group, and the outermost electrons are therefore further from the nucleus. The outermost electrons are also less strongly attracted to the nucleus, so they are more likely to be found at a greater distance from it.

The increase in atomic radii down the group is also due to the increase in the number of protons in the nucleus. The more protons there are in the nucleus, the stronger the attraction between the nucleus and the electrons. This means that the electrons are held more tightly to the nucleus, and so the atomic radius is smaller.

The combination of these two effects results in the increase in atomic radii down the group.