Plutonium-244 (Pu-244) is an isotope of plutonium with an atomic mass of 244 atomic mass units (amu), which is a whole number. The reason why Pu-244 has a whole number atomic mass while many other elements or isotopes do not is due to the concept of isotopic mass or atomic weight.
The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, taking into account their abundance. Most elements on the periodic table have multiple isotopes with different numbers of neutrons, resulting in non-whole number atomic masses.
However, Pu-244 is unique in that it is a long-lived radioactive isotope of plutonium with a half-life of about 80 million years. It is the most stable and abundant isotope of plutonium found in nature. Since Pu-244 is the only significant isotope of plutonium present in natural sources, its atomic mass is equal to its mass number, which is a whole number.
In contrast, for elements that have multiple isotopes with different abundances, the atomic mass is calculated as the weighted average of all the isotopes' masses, taking into account their relative abundance. This leads to atomic masses that are not whole numbers.