The smallest unit of matter is an atom. Atoms are the basic building blocks of matter and are composed of even smaller particles called protons, neutrons, and electrons. Protons and neutrons are found in the nucleus of an atom, while electrons orbit around the nucleus. However, it's worth noting that within the realm of subatomic particles, there are particles even smaller than atoms, such as quarks and leptons. These particles are considered elementary particles and are not composed of smaller components.