Final answer:
Noble Gases and Alkali Metals both have consistent valence electrons within their groups and exhibit uniform increments in their atomic and ionic sizes down the group, although they differ in reactivity and compounds they form.
Step-by-step explanation:
Similarities between Noble Gases and Alkali Metals
Although Noble Gases and Alkali Metals are quite different in their reactivity and typical compounds they form, they do share some similarities. Both groups contain elements that exhibit a uniform increment in their atomic and ionic radii down the group. Additionally, each group has a consistent number of valence electrons in their outermost electron shell respective to their group: Alkali Metals have one, while Noble Gases have eight (except for Helium, which has two).
The Alkali Metals (like lithium and sodium) readily lose their single valence electron to form cations with a +1 charge, a propitious trait due to their low first ionization energy and large atomic radii. In contrast, Noble Gases are characterized by their complete valence electron shells, which makes them unusually stable and nonreactive. Both groups exemplify the periodic trend of having full outer electron shells by either losing (Alkali Metals) or retaining (Noble Gases) electrons, an underlying principle that dictates the chemical behavior within a group.