code: def atom_fractions(self):
          r'''Dictionary of atom:fractional occurence of the elements in a
          chemical. Useful when performing element balances. For mass-fraction
          occurences, see :obj:`mass_fractions`.
  
          Examples
          --------
          >>> Chemical('Ammonium aluminium sulfate').atom_fractions
          {'H': 0.25, 'S': 0.125, 'Al': 0.0625, 'O': 0.5, 'N': 0.0625}
          '''
          if self.__atom_fractions:
              return self.__atom_fractions
          else:
              self.__atom_fractions = atom_fractions(self.atoms)
              return self.__atom_fractionf atom_fractions(self):
          r'''Dictionary of atom:fractional occurence of the elements in a
          chemical. Useful when performing element balances. For mass-fraction
          occurences, see :obj:`mass_fractions`.
  
          Examples
          --------
          >>> Chemical('Ammonium aluminium sulfate').atom_fractions
          {'H': 0.25, 'S': 0.125, 'Al': 0.0625, 'O': 0.5, 'N': 0.0625}
          '''
          if self.__atom_fractions:
              return self.__atom_fractions
          else:
              self.__atom_fractions = atom_fractions(self.atoms)
              return self.__atom_fractions
docs: r'''Dictionary of atom:fractional occurence of the elements in a
          chemical. Useful when performing element balances. For mass-fraction
          occurences, see :obj:`mass_fractions`.
  
          Examples
          --------
          >>> Chemical('Ammonium aluminium sulfate').atom_fractions
          {'H': 0.25, 'S': 0.125, 'Al': 0.0625, 'O': 0.5, 'N': 0.0625}