code: def _add_ideal_weight_percent_(elemental_array):
      
      t_mass = _calculate_total_mass_(elemental_array)
      for a in elemental_array:
          a["weight_percent"] = a["weight"] / t_mass * 100
      return elemental_array
docs: Adds the "weight_percent" property to each of the dictionaries in elemental_array
  
      :param elemental_array: an array of dictionaries containing information about the elements in the system
      :return: the appended elemental_array