"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.
Well, for the purposes of writing that story, we could
also postulate some, as of yet, undiscovered natural numbers between 0 and 1, existing only in that part of the universe ...