code: @Override
      public boolean showCarbon(IAtom atom, IAtomContainer container, RendererModel model) {
  
          Integer massNumber = atom.getMassNumber();
          if (massNumber != null) {
              try {
                  Integer expectedMassNumber = Isotopes.getInstance().getMajorIsotope(atom.getSymbol()).getMassNumber();
                  if (!massNumber.equals(expectedMassNumber)) return true;
              } catch (IOException e) {
                  logger.warn(e);
              }
          }
          return super.showCarbon(atom, container, model);
      }
docs: Returns true if the mass number of this element is set and not
  equal the mass number of the most abundant isotope of this element.
  
  @param  atom      {@link IAtom} which is being examined
  @param  container {@link IAtomContainer} of which the atom is part
  @param  model     the {@link RendererModel}
  @return true, when mass number information should be depicted