Unit 4 - Mark Rosengarten

Uranium Decay: U-238 is unstable and decays into more stable nucei. It takes 14 decay steps until a stable,
non-radioactive nucleus is finally reached. The daughter nuclide of one step becomes the parent nuclide of
the next:
PARENT NUCLIDE
DAUGHTER NUCLIDE
1) α) 238 92U 4 2He + 234 90 Th
2) β − ) 234 90Th 0 -1e +
234
91 Pa
3) β − ) 234 91Pa 0 -1e +
234
92 U
4) α)
5) α)
6) α)
7) α)
234
92 U 4 2He + 230 90Th
230
90 Th _____ + _______ OK, now YOU finish them off!
8) α)
9) β − )
10) β − )
11) α)
12) β − )
13) β − )
14) α)
The half-life of U-238, according to Reference Table N, is 4.51 X 10 9 years (4.51 billion years). The oldest rocks
on Earth consist of a 1:1 ratio of U-238 to Pb-206. This means that the Earth’s oldest rocks are 1 half-life of
uranium old. How old is the Earth? 4.51 billion years old! Looks pretty good for its age, doesn’t it?
The mode of decay can be found on Reference Table N.
© 2011, Mark Rosengarten R 8