What is the standard atomic weight of a Plutonium nucleus?

The standard atomic weight of a plutonium nucleus is 244 because it is determined by the average mass of all naturally occurring isotopes of plutonium. Plutonium has multiple isotopes, with different numbers of neutrons in their nuclei. The most common isotopes found in nature are plutonium-238, plutonium-239, and plutonium-240. These isotopes exist in different abundance, and their individual masses are taken into account to calculate the average atomic weight. By summing up the masses of each isotope, weighted by their relative abundance, an average atomic weight of 244 is obtained for plutonium.