code: private Integer massNumber(int atomicNumber, double exactMass) throws IOException {
          String symbol = PeriodicTable.getSymbol(atomicNumber);
          IIsotope isotope = Isotopes.getInstance().getIsotope(symbol, exactMass, 0.001);
          return isotope != null ? isotope.getMassNumber() : null;
      }
docs: Mass number for a atom with a given atomic number and exact mass.
  @param atomicNumber atomic number
  @param exactMass exact mass
  @return the mass number (or null) if no mass number was found
  @throws IOException isotope configuration could not be loaded