Uranium is far past that point. With 92 protons, it doesn’t matter how many neutrons you add to uranium—it will eventually decay. But there are two forms of uranium nuclei that will stick around for billions of years before they do: uranium-235 and uranium-238. The numbers refer to the total number of protons and neutrons in the nuclei: U-235 has 143 neutrons and 92 protons, for a total of 235. U-238 has 3 more neutrons, which makes it slightly heavier. But they’re both uranium: the chemical identity of an atomic nucleus is determined solely by the number of protons that it has.
Uranium 235 238
