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why does n/p ratio inrease?

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sapta:
why does n/p ratio inrease in the case of 92U238?it emits alpha particle.but why?

sdekivit:
the Uranium is under the stabilitybelt. So this means it has less neutrons than it should have to be stable ( or: too many protons). What must a particle to do to gain stabiltiy when it's under the stabilitybelt? it must lose what?

--> there are more neutrons in the nucleus (check mass number). So what effect will be larger, the loss of two neutrons or two protons ?

See for a detailed explanation the foolowing text:

http://www.amscopub.com/images/file/File_36.pdf

Mitch:
This is such a great question!!! But be warned great questions rarely have simple answers.

Your wondering why Uranium is unstable and why it emits alpha particles. But what is stabillity? The half life of U238 is ~109 years. Depending on who you talk to thats a very stable isotope. So, where do we draw the line on what is called "stable" or what isn't? When Uranium undergoes alpha decays it will emit helium nucleii and eventually become Lead 206. Lead 206 has a higher binding energy per nucleon. Meaning that the protons and neutrons are held more tightly in a  lead atom than an uranium atom. That being said, Uranium has a higher binding energy per nucleon than Helium! Helium is considered to be highly stable, why the contradiction? What would be a better way to measure nuclear stabillity? I'll leave the discussion open-ended so others can expand on it.

Grejak:

--- Quote from: sapta on August 04, 2005, 12:38:48 PM ---why does n/p ratio inrease in the case of 92U238?it emits alpha particle.but why?
--- End quote ---

As the atomic number of a nucleus increases, the coulomb repulsion bewtween the protons becomes more and more important.  In light nuclei, this repulsion between the protons is not very important, so you get a proton to neutron number approximately equal to one.  With heavier nuclei the protons begin to repel each other and a stable nucleus needs to have more and more neutrons in it in order to balance out the increased distance between the protons.  Since uranium is a heavy nucleus, the protons are highly repelled from each other and more neutrons fit in-between them.  This will give you a large n/p number.  

When uranium emits an alpha particle, it emits an equal number of protons and neutrons.  At this point it becomes a simple math problem.  If the original nucleus is 238U, then the nucleus has 92 protons and 146 neutrons, giving it an n/p ratio of 146/92 or 1.59.  After it emits an alpha particle, the new nucleus is 234Th which has 90 protons and 144 neutrons.  For this nucleus the n/p ratio is 144/90 or 1.6, a slight increase.

As a side note (and one the Mitch has asked me to add), even though the n/p ratio has increased the stability of the new nucleus has decreased.  Something cool to think about :)

sapta:

--- Quote ---the Uranium is under the stabilitybelt.
--- End quote ---

but (238-92)/92=1.586. So,how can it be below the stability belt? ???


--- Quote ---As a side note (and one the Mitch has asked me to add), even though the n/p ratio has increased the stability of the new nucleus has decreased.
--- End quote ---

why in the 1st place shud the ratio increase?wouldn't the daughter nuclide be farther away from the stability zone ???

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