docs: def get_isotopic_ratio(self, compound='', element=''):
          """returns the list of isotopes for the element of the compound defined with their stoichiometric values
  
          Parameters:
          ===========
          compound: string (default is empty). If empty, all the stoichiometric will be displayed
          element: string (default is same as compound).
  
          Raises:
          =======
          ValueError if element is not defined in the stack
          """
          _stack = self.stack
          compound = str(compound)
  
          if compound == '':
              _list_compounds = _stack.keys()
              list_all_dict = {}
              for _compound in _list_compounds:
                  _compound = str(_compound)
                  _list_element = _stack[_compound]['elements']
                  list_all_dict[_compound] = {}
                  for _element in _list_element:
                      list_all_dict[_compound][_element] = self.get_isotopic_ratio(
                          compound=_compound,
                          element=_element)
              return list_all_dict
  
          # checking compound is valid
          list_compounds = _stack.keys()
          if compound not in list_compounds:
              list_compounds_joined = ', '.join(list_compounds)
              raise ValueError("Compound '{}' could not be find in {}".format(compound, list_compounds_joined))
  
          # checking element is valid
          if element == '':
              # we assume that the element and compounds names matched
              element = compound
          list_element = _stack[compound].keys()
          if element not in list_element:
              list_element_joined = ', '.join(list_element)
              raise ValueError("Element '{}' should be any of those elements: {}".format(element, list_element_joined))
  
          list_istopes = _stack[compound][element]['isotopes']['list']
          list_ratio = _stack[compound][element]['isotopes']['isotopic_ratio']
          iso_ratio = zip(list_istopes, list_ratio)
  
          _stoichiometric_ratio = {}
          for _iso, _ratio in iso_ratio:
              _stoichiometric_ratio[_iso] = _ratio
  
          return _stoichiometric_ratio