Three isotopes of uranium occur in nature: U-238 (99.3% of all U in natural systems), U-235 (0.7%), and U-234 (0.005%).All of them are naturally-occuring radiogenic isotopes, and (as we have seen above) begin decay chains with geologically long half-lives.
The Table of Radionuclides documents the naturally-occurring radioisotopes.
Some isotopes decay and immediately produce a stable daughter product.
After another half-life, 25 units of the parent isotope will remain, and 75 units of the daughter product will have been produced.
After another half-life, 12.5 units of the parent isotope will remain, and 87.5 units of the daughter product will have been produced.
When an isotope emits an alpha particle, the resultant daughter product has an atomic number two units less than its parent's atomic number, and an atomic weight four units less than its parent's atomic weight.