diff --git a/src/pg_rad/isotopes.py b/src/pg_rad/isotopes.py
index 1d62d70..b9e3469 100644
--- a/src/pg_rad/isotopes.py
+++ b/src/pg_rad/isotopes.py
@@ -9,7 +9,6 @@ class Isotope:
 
         Args:
             name (str): _Full name (e.g. Caesium-137)._
-            symbol (str): _Shorthand symbol (e.g. 137Cs or Cs-137)_
             E (float): _Energy of the primary gamma in keV._
             b (float): _Branching ratio for the gamma at energy E._
         """