What is the standard atomic weight of a Plutonium nucleus?

The story behind the answer

244 is correct because plutonium has no stable isotopes, so IUPAC lists its “standard atomic weight” in brackets as the mass number of its longest-lived isotope: Pu‑244. That number counts protons plus neutrons (94 + 150 = 244). By contrast, 94 is plutonium’s atomic number (protons only), not its atomic weight.

Context tip: Pu‑244 has a very long half-life (~80 million years) and tiny primordial traces may still be found on Earth, while reactor-produced plutonium is usually Pu‑239 or Pu‑240. Memory aid: for synthetic elements like plutonium, look for the bracketed mass number—go with the big number (244), not the proton count (94).