Which of the following characteristics is associated with groups in the periodic table?

Groups in the periodic table comprise elements that share similar chemical properties primarily because of their similar electron configurations, particularly in their outermost shell. Elements within the same group have the same number of valence electrons, which greatly influences their reactivity and the types of chemical bonds they can form. For example, alkali metals (Group 1) all have one electron in their outer shell, leading to similar reactivity patterns and behavior in chemical reactions.

In contrast, while atomic mass may vary among elements in a group, it is not a defining characteristic, hence not applicable. The atomic radius can exhibit a general trend of increasing down a group; however, this is more of a trend than a characteristic of the group itself. Finally, reactivity can vary significantly within groups, especially for metals or nonmetals, making the statement about decreasing reactivity misleading. Thus, the defining characteristic aligning with group behavior is the similarity in chemical properties due to outer shell electron configuration.