The term "atom" historically referred to the smallest indivisible particle that made up matter. The concept of atoms dates back to ancient Greece, where philosophers such as Democritus proposed the idea that matter consisted of tiny, indivisible particles called atoms. However, with advancements in scientific understanding, we now know that atoms are not truly indivisible.
Atoms are composed of even smaller particles called subatomic particles, which include protons, neutrons, and electrons. Protons and neutrons are made up of smaller particles called quarks. Quarks are elementary particles and are considered to be the fundamental building blocks of matter.
While quarks are indeed smaller than protons and neutrons, they are not considered to be individual particles in the same way that protons and neutrons are. Quarks are always found in groups of two or three, tightly bound together by the strong nuclear force. When quarks combine, they form composite particles such as protons and neutrons.
So, while atoms were once thought to be the smallest particles, our understanding of the subatomic world has evolved to recognize the existence of quarks and other subatomic particles. The term "atom" is still used to refer to the basic unit of matter, but we now know that atoms are composed of even smaller constituents.