public Isotope getIsotope(int massNumber) {
   Iterator<Isotope> isotopeIterator = isotopes.iterator();
   Isotope isotope;
   while (isotopeIterator.hasNext()) {
     isotope = isotopeIterator.next();
     if (isotope.getMassNumber() == massNumber) {
       return isotope;
     }
   }
   return null;
 }