def atoms(self, symbol):
        """Return the number of type 'symbol' atoms in the molecule

           Raises:
              KeyError - if the symbol is not in atomic periodic table

        """
        molTest = Molecule(symbol)
        molTest.check_symbols()

        total_symbols = 0
        total_symbols_loop = 0
        molecule_list = self._tokentree()
        for item in molecule_list:
            if len(item[0]) > 1:
                for x in item[0]:
                    if x[0] == symbol:
                        total_symbols_loop = 0
                        total_symbols_loop = total_symbols_loop + x[1]
                total_symbols = total_symbols_loop * item[1]
            else:
                if item[0] == symbol:
                    total_symbols = total_symbols + item[1]
        return total_symbols