Why Does Rb87 Decay to Sr87, Not Rb85?

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In summary, decay is a process of losing protons and electrons and the result is a different element.
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mrhagerty
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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
 
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  • #2
mrhagerty said:
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.
 
  • #3
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.
 
  • #4
Thank you all for your timely replies. Your explanations clear up the confusion.
 

1. Why does Rb87 decay to Sr87?

Rb87, or Rubidium-87, is a naturally occurring radioactive isotope. This means that it tends to break down or decay into more stable forms over time. In the case of Rb87, it decays into Sr87, or Strontium-87, through a process called beta decay.

2. What is the difference between Rb87 and Rb85?

Rb87 and Rb85 are both isotopes of the element Rubidium. The difference between them lies in their atomic structures. Rb87 has 87 protons and 87 neutrons, while Rb85 has 85 protons and 85 neutrons. This difference in the number of neutrons makes them behave differently in terms of stability and decay.

3. How long does it take for Rb87 to decay to Sr87?

The half-life of Rb87 is approximately 49 billion years, which means that it takes 49 billion years for half of the Rb87 atoms in a sample to decay into Sr87. This time period is incredibly long compared to the age of the Earth, which is estimated to be around 4.5 billion years.

4. Why does Rb87 decay to Sr87 more often than Rb85?

The half-life of Rb87 is longer than that of Rb85, meaning that it takes longer for Rb87 to decay into Sr87. Additionally, Rb87 has a higher energy state than Rb85, making it more likely to undergo beta decay and transform into a more stable isotope.

5. What are the implications of Rb87 decaying to Sr87 in terms of radiometric dating?

Rb87 decaying to Sr87 is commonly used in radiometric dating to determine the age of rocks and other geological materials. By measuring the ratio of Rb87 to Sr87 in a sample and knowing the half-life of Rb87, scientists can calculate the age of the sample. This method is particularly useful for dating older materials, as the half-life of Rb87 is much longer than other commonly used isotopes.

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