What is a synonymous term for a polyatomic ion?

The term "radical" is synonymous with a polyatomic ion because both refer to a group of atoms that carry a charge, which can be positive or negative. A polyatomic ion consists of two or more atoms bonded together that have an overall charge due to the loss or gain of electrons. This aligns closely with the concept of a radical, which often describes a similar group of atoms that have unpaired electrons and may also possess a charge.

On the other hand, a "molecule" typically refers to a stable aggregation of atoms held together by covalent bonds and generally does not imply an overall charge. An "atom" is the smallest unit of an element and does not consist of multiple atoms, while a "compound" is a substance formed when different elements chemically bond together, which can be either ionic or covalent but does not specifically imply a charged entity like a polyatomic ion or radical does. Therefore, "radical" is the correct answer as it captures the essential characteristic of polyatomic ions in terms of their charged nature and multi-atomic composition.