How is an atom defined?

An atom is defined as the basic unit of a chemical element because it represents the smallest unit of an element that retains its unique properties and characteristics. Atoms consist of protons, neutrons, and electrons, which together define the identity of the element, whether it be hydrogen, helium, or any other element in the periodic table. This foundational role of atoms is crucial in understanding not only elements but also the compounds they form when they bond with other atoms.

In contrast, the other choices do not accurately reflect the definition of an atom. Atoms are not simply the smallest particles of a compound, as they are fundamental units that can exist independently or combine with other atoms to form compounds. A collection of molecules refers to multiple groups of atoms bonded together, which is a larger structure than a single atom itself. Finally, while ions are atoms or molecules that have gained or lost electrons and thus have a charge, the definition of an atom encompasses neutral particles as well, making it broader than just ions or charged forms of matter.