Here's why:

* Rubidium (Rb) is an alkali metal in group 1 of the periodic table. Alkali metals have a +1 charge when they form ionic compounds.

* Hydrogen (H) is a nonmetal and typically forms an anion with a -1 charge (hydride ion, H⁻).

To form a neutral compound, one rubidium ion (Rb⁺) combines with one hydride ion (H⁻), resulting in the formula RbH.