What is the smallest unit of a chemical element?

The smallest unit of a chemical element is an atom. Atoms are the fundamental building blocks of matter and retain the properties of the element they represent. Each atom consists of a nucleus, containing protons and neutrons, surrounded by electrons that orbit the nucleus. By definition, an atom is the smallest part of an element that can participate in a chemical reaction while maintaining the chemical identity of that element.

Molecules, ions, and compounds are larger structures and arise from the combinations of atoms. A molecule consists of two or more atoms bonded together, an ion is an atom or a molecule that has lost or gained one or more electrons, and a compound is a substance formed when two or more different types of atoms are bonded together. Therefore, while these other entities are essential in understanding chemical interactions and compounds, they do not qualify as the smallest unit of a chemical element, making the atom the correct answer in this context.