Why are Some Daughter Isotopes More Unstable Than Their Parent Isotope?

Why are some daughter isotopes more unstable than their parent isotope? For example, the daughter isotope of uranium-238, which is thorium-234, only has a half-life of 24 days compared to uranium-238's half-life of 4.5 billion years. This can be seen again and again in uranium-238's decay chain. I would have thought that each link in the decay chain would result in a more stable form.
- Stephen (age 29)
Canada

Hello Stephen,

As you probably know Uranium 238 has a torturous path through 13 different isotopes on its way to Lead 206 which is stable.  See:    for details.   Each link in  the chain has a different lifetime ranging from milliseconds to 1600 years.   The speed of a particular transitiion depends on several things, the main ones are the quantum mechanical overlap of the before and after wave functions and the available left over kinetic energy arising from the mass difference between in initial and final state.  In the case of U to Th the available kinetic energy is quite small hence the longish lifetime.  

 

LeeH

(published on 12/29/2013)