Why Does Rb87 Decay to Sr87, Not Rb85?

[mrhagerty]

I'm new here and have a very basic question that I haven't been able to see answered here or elsewhere, so forgive the banality if this seems trivial.

I understand Rubidium has a "normal" atomic weight of 85 and often exists as its isotope Rb87.

When Rb87 decays, why doesn't it decay to Rb85 instead of Sr87?

My understanding is that decay is effected through alpha, beta or gamma losses to the atom. The result can be a loss of protons and electrons that form the daughter element (U238 (#92) to Pb208 (#82).

But Rb to Sr retains the same weight (87 - 87), with an actual gain of electrons (37 - 38).

I'm sure I'm missing something very basic but that's why I'm here - to learn.

Mike

 

[mfb]

I understand Rubidium has a "normal" atomic weight of 85 and often exists as its isotope Rb87.

About 72% with 85 and 28% with 87.

When Rb87 decays, why doesn't it decay to Rb85 instead of Sr87?

Why should it?
It is impossible: This would mean the emission of neutrons, but there is not enough energy for them to escape. The conversion of neutrons to protons (plus electron plus neutrino) is possible.

My understanding is that decay is effected through alpha, beta or gamma losses to the atom.

Those are the most common decay modes, right.

The result can be a loss of protons and electrons that form the daughter element (U238 (#92) to Pb208 (#82).

Ignore the electrons, they are not relevant (for neutral atoms, they just follow whatever the nucleus does). Alpha decays mainly mean a loss of nucleons. Due to beta decays, protons and neutrons can convert into each other.

But Rb to Sr retains the same weight (87 - 87), with an actual gain of electrons (37 - 38).

Yes, this is a result of a beta decay.

 

[mathman]

Rb 87 decays by emitting an electron from the nucleus (beta decay) Therefore the At. No. changes from 37 to 38.

Alpha and beta decay, as well as electron capture and positron emission, are all nuclear reactions.

 

[mrhagerty]

Thank you all for your timely replies. Your explanations clear up the confusion.