Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
Praseodymium
All other praseodymium isotopes have half-lives under a day (and most under a minute), with the single exception of Pr with a half-life of 13.6 days. Both Pr and Pr occur as fission products of uranium. The primary decay mode of isotopes lighter than Pr is inverse beta decay or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium.