Here's how to figure that out:
* Rubidium is a Group 1 element (alkali metal) with a +1 charge (Rb⁺).
* Phosphate is a polyatomic ion with a -3 charge (PO₄³⁻).
To balance the charges, you need three rubidium ions (3 x +1 = +3) to combine with one phosphate ion (-3). This gives you the formula Rb₃PO₄.
