An atom is the smallest quantity of any given element.
An atom includes a nucleus (which includes protons and neutrons) around which are located the electrons. That's that specific assembly of nucleus and surrounding electrons that form an atom and define which element it is.
So,
electron: negative charge in orbit of the nucleus
proton: positively-charged particle inside the atomic nucleus
neutron: neutral particle inside the atomic nucleus
atom: ensemble of the nucleus and electrons, defining a given element.
