code: public ValidationReport validateIsotopeExistence(IIsotope isotope) {
          ValidationReport report = new ValidationReport();
          ValidationTest isotopeExists = new ValidationTest(isotope,
                  "Isotope with this mass number is not known for this element.");
          try {
              IsotopeFactory isotopeFac = Isotopes.getInstance();
              IIsotope[] isotopes = isotopeFac.getIsotopes(isotope.getSymbol());
              if (isotope.getMassNumber() != null &&
                  isotope.getMassNumber() != 0) {
                  boolean foundKnownIsotope = false;
                  for (int i = 0; i < isotopes.length; i++) {
                      if (Objects.equals(isotopes[i].getMassNumber(), isotope.getMassNumber())) {
                          foundKnownIsotope = true;
                      }
                  }
                  if (!foundKnownIsotope) {
                      report.addError(isotopeExists);
                  } else {
                      report.addOK(isotopeExists);
                  }
              } else {
                  // isotopic is unspecified
                  report.addOK(isotopeExists);
              }
          } catch (Exception exception) {
              // too bad...
          }
          return report;
      }
docs: the Isotope tests