If atoms are made of smaller parts such as electrons, why are atoms considered the basic unit of matter?

Atoms are considered the basic unit of matter because it is atoms that make up the elements which in turn combine to form compounds that exist in all states. It is true that atoms are made up of electrons and neutrons and protons, but they are not useful on their own individual level. Like, you don't hear anyone say that "the particles that make up atoms are the basic unit of matter". 

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Answer ozone (o3) is made of three atoms of oxygen, it is a molecule;