code: def as_df():
      
      records = []
      for sym, ele in vars(_this).items():
          if sym not in ["Element", "Isotope"] and not sym.startswith("_"):
              for k, v in vars(ele).items():
                  if k.startswith("_") and k[1].isdigit():
                      records.append({kk: vv for kk, vv in vars(v).items() if not kk.startswith("_")})
      return _DF.from_records(records)
docs: Return a dataframe of isotopes.