Final answer:
Rubidium is expected to form a positive ion, Rb+, because it is an alkali metal with one valence electron that it easily loses to achieve a noble gas electron configuration.
Step-by-step explanation:
The monatomic ion that rubidium (Rb) is expected to form is Rb+. Rubidium is a highly reactive alkali metal, located in group 1 of the periodic table, which characteristically forms positive ions. This behavior is a result of rubidium having one valence electron that it readily loses to achieve a stable noble gas electron configuration, specifically imitating krypton.
In binary ionic compounds, metals generally form positive ions, while nonmetals tend to form negative ions. Rubidium, with its low first ionization energy as indicated by its position in the periodic table, will most likely form the Rb+ ion upon losing that single valence electron.
This can also be corroborated by rubidium's electronic configuration. Rubidium's abbreviated electron configuration is [Kr] 5s1, suggesting that it will lose one electron and form a +1 charge to achieve the [Kr] configuration. Therefore, the expectation of rubidium forming a Rb+ ion in binary ionic compounds is consistent with its properties and placement in the periodic table.