code: public static void clearMajorIsotopes(IAtomContainer mol) {
          for (IAtom atom : mol.atoms())
              if (isMajor(atom)) {
                  atom.setMassNumber(null);
                  atom.setExactMass(null);
                  atom.setNaturalAbundance(null);
              }
      }
docs: Clear the isotope information from atoms that are major isotopes (e.g.
  <sup>12</sup>C, <sup>1</sup>H, etc).
  @param mol the molecule