code: private static int atomicNumber(IAtom atom) {
          Integer elem = atom.getAtomicNumber();
          if (elem != null) return elem;
          if (atom instanceof IPseudoAtom) return 0;
          throw new NullPointerException("a non-pseudoatom had unset atomic number");
      }
docs: Access atomic number of atom defaulting to 0 for pseudo atoms.
  
  @param atom an atom
  @return the atomic number
  @throws NullPointerException the atom was non-pseudo at did not have an
  atomic number