Final answer:
The polyatomic ion in the compound K₂HPO₄ is HPO₄²⁻ with a charge of -2.
Step-by-step explanation:
The polyatomic ion in the compound K₂HPO₄ is HPO₄²⁻. It has a charge of -2 since dihydrogen phosphate (HPO₄²⁻) is combined with two K+ ions. Therefore, the compound is written as K₂HPO₄, with the polyatomic ion HPO₄²⁻ included along with its charge.