Understanding Neutrality in Alpha/Beta Decay

[troy611]

I have the following confusion:

In an alpha or beta decay, there's a change in the nucleus, i.e. there's a change in number of protons and neutrons, but there's no change in the number of electrons. So, how is it that the product is neutral?

To make this more clear,

Suppose U-238 (with 92 protons, 92 electrons, 146 neutrons) decays(alpha decay, i.e. helium nuclei is emitted) into Th-234 (90 protons, 144 neutrons). The confusion is: to be neutral, Th formed should have 90 electrons, but there are 92 electrons to start with...what happens to the other 2 electrons?

This applies to other decays as well...thank u for any help

 

[Bob S]

Suppose U-238 (with 92 protons, 92 electrons, 146 neutrons) decays(alpha decay, i.e. helium nuclei is emitted) into Th-234 (90 protons, 144 neutrons). The confusion is: to be neutral, Th formed should have 90 electrons, but there are 92 electrons to start with...what happens to the other 2 electrons?

If this were in metallic U-238, the alpha particle would stop, find two electrons, and become a helium atom. Simultaneously, the Th-234 would release the two extra electrons it doesn't want. If this were in an insulator, there may be dislocations (charge centers), because the helium atom will be displaced from the Th-234..

Bob S

 

[bcrowell]

Suppose U-238 (with 92 protons, 92 electrons, 146 neutrons) decays(alpha decay, i.e. helium nuclei is emitted) into Th-234 (90 protons, 144 neutrons). The confusion is: to be neutral, Th formed should have 90 electrons, but there are 92 electrons to start with...what happens to the other 2 electrons?

No electrons were created or destroyed. You had 92 electrons to start with, and there are still 92 electrons somewhere.

 

[troy611]

@Bob S...so you mean in this particular decay, no alpha particles are formed?
@bcrowell...they can't be destroyed, but that is my question, they are somewhere, but where are they?

 

[Bob S]

@Bob S...so you mean in this particular decay, no alpha particles are formed?

In this type of decay, alpha particles certainly are formed, and they move a few (or tens or hundreds) of microns before they stop. Of all the elements, helium (of which the alpha particle is the nucleus) has the highest ionization energy (~ 24 eV) and so the helium ion has the highest affinity for grabbing electrons. When the alpha particle stops, it will steal two electrons from the U-238 and become a helium molecule in the U-238.

Bob S

 

[troy611]

ok...now i understood it regarding alpha decay

but what about beta decay?

for example,
P (17 neutrons, 15 protons, 15 electrons) beta(-) decays into S (16 neutrons, 16 protons).
1 electron, 1 proton(which stays in the nucleus), and 1 anti neutrino are formed. In my textbooks and all, its given that the electron formed escapes.

now, for neutrality, S should have 16 electrons, but the atom has only 15 electrons(the one electron formed has escaped), does that mean the S atom formed is positively charged?

ps: i m sorry, it may be becoming boring for u...but i really have this confusion...thanx for helping out on the alpha decay.

 

[Astronuc]

When a beta particle escapes a nucleus (n -> p), then the charge increases from Z to Z+1, e.g., P(Z=15) to S(Z=16). The beta particle escapes that nucleus/atom, but it does not travel far. May be mms' or cm's. Meanwhile, the Z+1 nucleus attracts an electron, and then there is a cascade of electrons transferred until somewhere the beta particle (an electron) is captured by an ion and becomes a neutralized atom.