Here's why:

* Ionic bonds form between a metal and a nonmetal, where one atom donates an electron to another, creating oppositely charged ions that attract.

* Covalent bonds form between two nonmetals, where atoms share electrons.

Polyatomic ions are groups of two or more atoms held together by covalent bonds, but they carry an overall charge, making them act as a single unit in ionic compounds.

Examples:

* Nitrate ion (NO₃⁻): The nitrogen and oxygen atoms are held together by covalent bonds, but the ion as a whole carries a negative charge. This allows it to form ionic bonds with metal cations like sodium in sodium nitrate (NaNO₃).

* Ammonium ion (NH₄⁺): Nitrogen and hydrogen are covalently bonded, but the ion has a positive charge, allowing it to form ionic bonds with anions like chloride in ammonium chloride (NH₄Cl).

These polyatomic ions bridge the gap between purely ionic and purely covalent compounds.