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This question already has an answer here:

In alpha decay, a $\text{He}$ nucleus is emited along with a daughter nuclide. Now suppose $\text{U}$ with atomic number 92 and atomic mass 238 emits an alpha particle and a daughter nuclide is formed with atomic number 90 and atomic mass 234, then my question is:

  • what happened with the electrons in this process? Some say that this is only a nuclear reaction, so electrons are not concerned. If so, then why do we say that half of the atoms decay at half-life? Should we not say that half of the nucleus decayed?
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marked as duplicate by Kyle Kanos, ACuriousMind, rob, Emilio Pisanty, John Rennie May 25 '15 at 6:46

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ U238 decaying by alpha emission does end up with mass 234. What about the electrons? Well, they are pretty much ignored given the energies released in nuclear decay. From a sold film of material, most alphas will come out ionized (Geiger and Marsden used a natural alpha source in their experiments). The odd left over electrons will eventually find a new home. $\endgroup$ – Jon Custer May 24 '15 at 18:23
  • $\begingroup$ @BintEHawaGuijar I have attemptet to correct spelling and grammar in your question. Please check that it is still telling the story you want (and double-check the values, as another comment mentions.) $\endgroup$ – Steeven May 24 '15 at 23:43
  • $\begingroup$ I deleted my comment because Bill N edited the question. It was so badly written that I read "U" as you ( shorthand in phones) $\endgroup$ – anna v May 25 '15 at 3:03
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    $\begingroup$ possible duplicate of What Happens to electrons after Alpha Decay and Nuclear Fission? $\endgroup$ – rob May 25 '15 at 4:55
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The electrons stay with the daughter nucleus. A good way to see this is by imagining the decay. Imagine an atom. A helium nucleus shoots out. By the definition of a nucleus, it has no electrons. Therefore, the electrons must still be on the atom.

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  • $\begingroup$ This might be a better answer if you expanded on the single sentence. $\endgroup$ – Kyle Kanos May 24 '15 at 23:20
  • $\begingroup$ And then at least one of the electrons departs the atom because -2e ions are not stable at all. (I'd love it if they were even metastable for long enough to get to the terminal of a tandem accelerator, but, alas, they are not.) $\endgroup$ – Jon Custer May 25 '15 at 0:51
  • $\begingroup$ @JonCuster The neutralization of the negative heavy ion left after the decay is a much slower and less energetic process than alpha emission. I haven't looked in the literature but I expect the number of singly-charged alphas is quite small. $\endgroup$ – rob May 25 '15 at 5:00
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what happened with the electrons in this process?

After fission, the potential that bound the 92 electrons changes. The alpha has too much kinetic energy and cannot trap the two electrons it needs. It will pick up electrons at it comes to rest in the material or the air. The remaining, now Thorium, nucleus reorganizes and the electrons are bound in the new energy levels. The two extra will be superfluous . One might bind in a higher level resonant line, and create a negative ion, the other will go to another nucleus and create a negative ion, or to the conduction band, Uranium is a metal.

Some say that this is only a nuclear reaction, so electrons are not concerned.

All physicists call fission a nuclear reaction.

If so, then why do we say that half of the atoms decay at half-life? Should we not say that half of the nucleus decayed?

The half life describing the fission characterizes the atomic number, i.e. the number of protons and neutrons of the nucleus. When that changes it is called a decay, a new atom is forced into existence, the old decayed. It is the nucleus that defines the atom as a whole.

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