What is an atom?

An atom is defined as the smallest unit of an element that retains the properties of that element. This means that even at the atomic level, the characteristics that define the element, such as its chemical behavior and identity, are preserved. For instance, a single hydrogen atom still exhibits the properties of hydrogen, such as its ability to form bonds or react in chemical reactions.

Each atom is made up of protons, neutrons, and electrons, where protons and neutrons form the nucleus at the center, and electrons orbit around this nucleus. This intricate arrangement allows for the distinct properties of various elements, which are fundamental to the study of chemistry.

In contrast, the other options do not accurately describe an atom. Descriptions of atoms as the smallest unit of a compound or as a particle made up of protons and neutrons only do not capture the full definition and significance of an atom in chemistry. Moreover, defining an atom as a collection of molecules suggests an incorrect structure, as a molecule is made up of two or more atoms bonded together, not the other way around. This clarifies why the chosen answer encapsulates the essence of an atom within the broader context of chemical elements and their properties.