But then it's neither an atom or normal matter. Atom implies a singular unit of Protons, Neutron and Electrons as Chris said.
"Protium" or Hydrogen-1, is the lightest possible atom. H+ (the Electron stripped away) is simply a Proton on it's own and no longer an atom.
Or simply, less than 1 is 0, ie nothing.
I know other exotic matter forms do behave in similar ways but they aren't by definition an atom, and usually are incredibly short lived or not compatible with normal matter in a safe way.