If Sr90 begins with U238 why does Sr90 is unstable and decays via beta emmission?

Answer 1

The forces that hold the nucleus of an atom together may not be perfectly understood, but they do behave in fairly regular and predictable ways. They try and reach a stable point, but they only have a few ways of getting there.

And that's the point. Sometimes there isn't really a way for an unstable nucleus to become stable in just one decay, or even in a dozen decays. Many radioactive materials (like uranium) turn into one element after another until they finally reach stability.

The starting uranium atom in this case splits asymmetrically into two halves with mass numbers from 90 to 140 generally. About 6% of the time one of the products is a Strontium-90 atom.

Now, just because Strontium-90 was produced, it hardly means it's automatically stable... that's just how the nucleus happened to break up. Strontium always decays again into Yttrium-90 which in turn always decays into Zirconium-90, which is finally stable.

Hope that helps!