code: public IIsotope getMajorIsotope(int elem) {
          IIsotope major = null;
          if (this.majorIsotope[elem] != null) {
              return clone(this.majorIsotope[elem]);
          }
          List<IIsotope> isotopes = this.isotopes[elem];
          if (isotopes != null) {
              for (IIsotope isotope : isotopes) {
                  if (isotope.getNaturalAbundance() <= 0)
                      continue;
                  if (major == null ||
                      isotope.getNaturalAbundance() > major.getNaturalAbundance()) {
                      major = isotope;
                  }
              }
              if (major != null)
                  this.majorIsotope[elem] = major;
              else
                  logger.error("Could not find major isotope for: ", elem);
          }
          return clone(major);
      }
docs: Returns the most abundant (major) isotope with a given atomic number.
  Note that some high mass elements do not have a major isotopes
  (0% abundance) and this method will return null for those.
  
  @param  elem  The atomicNumber for which an isotope is to be returned
  @return       The isotope corresponding to the given atomic number
  
  @see #getMajorIsotope(String symbol)