BaSO₄ is not a polyatomic ion, it is an ionic compound formed from the barium cation (Ba²⁺) and the sulfate anion (SO₄²⁻).

Here's why:

* Polyatomic ions are groups of atoms that act as a single unit with a charge, like the sulfate ion (SO₄²⁻).

* Ionic compounds are formed by the electrostatic attraction between positively charged cations and negatively charged anions.

Therefore, the formula BaSO₄ represents the ionic compound barium sulfate.