Rubidium is an alkali metal. When alkali metals react with halogens, they form binary ionic halides. Here, the reaction between rubidium (Rb) and chlorine (Cl) is given as follows:

$$2Rb_{(s)}+Cl_{2(g)}\rightarrow 2RbCl_{(s)}$$

In this reaction, rubidium loses one electron from its outermost shell to the chlorine atom, resulting in the formation of a positive rubidium ion (Rb+) and a negative chloride ion (Cl-). These ions combine electrostatically to form rubidium chloride (RbCl), which is a stable ionic compound.