code: public double getNaturalMass(int atomicNum) {
          List<IIsotope> isotopes = this.isotopes[atomicNum];
          if (isotopes == null)
              return 0;
          double summedAbundances = 0;
          double summedWeightedAbundances = 0;
          double getNaturalMass = 0;
          for (IIsotope isotope : isotopes) {
              summedAbundances += isotope.getNaturalAbundance();
              summedWeightedAbundances += isotope.getNaturalAbundance() * isotope.getExactMass();
              getNaturalMass = summedWeightedAbundances / summedAbundances;
          }
          return getNaturalMass;
      }
docs: Gets the natural mass of this element, defined as average of masses of
  isotopes, weighted by abundance.
  
  @param atomicNum the element in question
  @return The natural mass value