Rubidium most commonly forms a cation, specifically the Rb+ ion. As an alkali metal located in Group 1 of the periodic table, rubidium has a single electron in its outermost shell. In order to achieve a more stable electron configuration, it tends to lose this electron.
When rubidium loses its outer electron, it results in a positively charged ion, or cation. This is characterized by having more protons than electrons, leading to its positive charge. In contrast, anions are formed when atoms gain electrons, which rubidium does not typically do due to its position on the periodic table and its desire to lose its single valence electron.