What is the formula for rubidium chloride?

The formula for rubidium chloride is $\text{RbCl}$.
Rubidium $\text{Rb}$ is an alkali metal. It wants to lose one electron to become stable. Chlorine $\text{Cl}$ is a halogen. It wants to gain and electron to become stable. So if we add the two together we get:
$\text{Rb"^+ + "Cl"^- -> "RbCl}$.