The polyatomic compound in Rb₃PO₄ is phosphate, represented by the formula PO₄⁻³.

Here's why:

* Rb represents rubidium, an alkali metal. Alkali metals typically form +1 ions (Rb⁺).

* PO₄⁻³ represents the phosphate ion, a polyatomic anion with a -3 charge.

To balance the charges and form a neutral compound, we need three rubidium ions (3 x +1 = +3) to combine with one phosphate ion (-3).

Therefore, the chemical formula for the compound is Rb₃PO₄.