Here's why:

* Rubidium (Rb) is an alkali metal in group 1 of the periodic table. It has a +1 charge when it forms ions.

* Peroxide (O₂²⁻) is a polyatomic ion with a -2 charge.

To balance the charges, you need two rubidium ions (2 x +1 = +2) to combine with one peroxide ion (-2).