What is meant by the term "polyatomic ion"?

The term "polyatomic ion" refers to a charged species that consists of two or more atoms that are covalently bonded together. This cluster of atoms behaves as a single unit in chemical reactions and has an overall charge because the total number of protons does not equal the total number of electrons. For instance, common polyatomic ions include sulfate (SO₄²⁻) and nitrate (NO₃⁻), which both contain multiple atoms and carry a charge.

In contrast, a single atom with a charge is simply a monatomic ion, while the mention of independence points to entities not relevant to the nature of polyatomic ions, which are often attached to other ions in compounds. Lastly, the definition encompassing only carbon and hydrogen atoms does not pertain to the concept of ions and is too narrow, as polyatomic ions can contain various elements. Thus, the selected choice accurately captures the essence of what constitutes a polyatomic ion in chemistry.