@property<EOL><INDENT>def atom_fractions(self):<DEDENT>
if self.__atom_fractions:<EOL><INDENT>return self.__atom_fractions<EOL><DEDENT>else:<EOL><INDENT>self.__atom_fractions = atom_fractions(self.atoms)<EOL>return self.__atom_fractions<EOL><DEDENT>
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}