What is the smallest particle that takes part in a chemical reaction?

The correct response identifies the atom as the smallest particle that participates in a chemical reaction. Atoms are the fundamental building blocks of matter and consist of protons, neutrons, and electrons. When chemical reactions occur, it is the atoms that are rearranged, combined, or separated, resulting in the formation of new substances.

In contrast, while molecules (which are formed from two or more atoms bonded together) can take part in chemical reactions, they are not the smallest units since they consist of multiple atoms. Compounds are composed of two or more different types of atoms bonded together, and thus also consist of multiple atoms. Elements are pure substances made up entirely of one type of atom, and while they can involve individual atoms, the term isn't specifically about the reactivity of those atoms in a chemical sense.

Understanding that atoms are indivisible in terms of chemical reactions allows us to grasp the basic principles of chemistry and how substances interact at their most fundamental level.