The polyatomic ion in the compound Na₂O₂ is peroxide, with the formula O₂⁻².

Here's why:

* Na represents the element sodium, which typically forms a +1 cation (Na⁺).

* O represents the element oxygen.

* The subscript 2 after O indicates that there are two oxygen atoms in the ion.

* The overall charge of the ion is -2 to balance the +2 charge from the two sodium ions.

Therefore, the compound Na₂O₂ is called sodium peroxide.