What term describes a group of atoms acting as a single unit in a chemical reaction?

The term that describes a group of atoms acting as a single unit in a chemical reaction is a molecule. A molecule forms when two or more atoms bond together, either of the same or different elements, and the entire assembly behaves as a distinct entity. This is fundamental in understanding chemical reactions, as molecules can interact, combine, or break apart during reactions while maintaining their identity as a specific chemical structure.

In chemical terms, a radical specifically refers to an atom, molecule, or ion that has unpaired electrons, which makes it highly reactive but does not necessarily imply it acts as a single unit like a molecule does. An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge, but it does not specifically represent groups of atoms functioning together in the same way. A compound, while it consists of two or more different elements bonded together, refers specifically to a particular type of molecule made of differing atoms, rather than just a collective term for a group of atoms in general.

Ultimately, understanding these distinctions clarifies why "molecule" is the correct choice in the context of a group of atoms acting in unison during a reaction.