code: private int element(IAtom atom) {
          Integer element = atom.getAtomicNumber();
          if (element != null) return element;
          if (atom instanceof IPseudoAtom) return 0;
          throw new IllegalArgumentException("Aromaiticty model requires atomic numbers to be set");
      }
docs: Get the atomic number as an non-null integer value. Although pseudo atoms
  are not considered by this model the pseudo atoms are intercepted to have
  non-null atomic number (defaults to 0).
  
  @param atom atom to get the element from
  @return the formal charge