Hello can someone please help me identify each element as polyatomic or monatomic Thanks.

Answer:
Monoatomic ions: Li⁺, Be²⁺, Cr²⁺, Pb²⁺, Cl⁻, O²⁻
Polyatomic ions: H₃O⁺, OH⁻, SO₄²⁻, Hg₂²⁺
Explanation:
Polyatomic ions have more than one atom. Monoatomic ions have only one atom.