Does having more neutrons in an isotope make it more or les stable?

In summary, the conversation discusses the stability of isotopes and how the number of neutrons can affect it. It is explained that isotopes can have the same number of protons and neutrons, and that the stability of isotopes varies with the number of protons. The conversation also mentions that adding neutrons can make a nucleus more stable, but there is a limit to this and adding too many neutrons can actually make it more unstable. It is also noted that for elements with a high atomic number, there may not be any stable isotopes. The conversation ends with a question about whether the half life of an isotope can be accurately predicted.
  • #1
Tangeton
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Does more neutrons in an isotope make it more or less stable? I got two contradicting sources. I am also quite confused overall about the whole isotope business.

The definition of an isotope is that it is an atom with the same number of protons but different number of neutrons.

For example, carbon isotopes go from 8 to 22 as far as wiki says (https://en.wikipedia.org/wiki/Isotopes_of_carbon) and it also says 12C and 13C are both stable, and all others are unstable. But, first of all, isn't 12C not an isotope, since the number of protons is the same as number of neutrons?

Secondly, it seems that there are unstable 8C, 9C and so forth and then 14C over to 22C must be unstable as well. So it seems to me that greater the number of neutrons doesn't exactly mean either more or less unstable, but rather that the closer the number of neutrons to protons the more stable the atom. Why would these books/internet/teachers try to say either of these incorrect statements?

Thanks for help.
 
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  • #2
Tangeton said:
But, first of all, isn't 12C not an isotope, since the number of protons is the same as number of neutrons?
It is a carbon isotope. Isotope does not mean it cannot have the same number of protons and neutrons. Just that nuclei with a given number of protons are isotopes of the same element.

What isotopes are stable varies with the number of protons. For relatively light elements, you need roughly the same number of protons and neutrons. For heavier elements there is usually more neutrons in the stable isotopes.

Without you actually referencing a text, it is impossible to say whether it is the statement or your interpretation of it that id incorrect.
 
  • #3
Tangeton said:
The definition of an isotope is that it is an atom with the same number of protons but different number of neutrons.
you can always call all the elements isotopes... the only difference between isotopes of an element is the number of neutrons... so 12C and 13C are carbon-isotopes; differing only to the number of neutrons.

Tangeton said:
But, first of all, isn't 12C not an isotope, since the number of protons is the same as number of neutrons?
my above comment answers it.

Tangeton said:
Secondly, it seems that there are unstable 8C, 9C and so forth and then 14C over to 22C must be unstable as well. So it seems to me that greater the number of neutrons doesn't exactly mean either more or less unstable, but rather that the closer the number of neutrons to protons the more stable the atom. Why would these books/internet/teachers try to say either of these incorrect statements?

In general the number of neutrons act as a glue for heavier nuclei... That is the reason of the trend in the Nuclear Stability Curve (http://butane.chem.uiuc.edu/pshapley/GenChem1/L5/graph.gif) where you can see that the curve has a trend to move away from the Nprotons=Nneutrons.
Adding neutrons makes the strong interaction stronger and the element more stable. The fewer neutron nuclei then are unstable because protons "don't really like" coexisting together alone. Adding blindly neutrons is not helping though, because the extra neutrons [being fermions] have to keep occupying higher energy levels of the nucleus [it's the reason of the 'asymmety' term in the Semi-empirical formula]... in other words you make your nucleus unstable with respect to beta- decay.
For heavier nuclei the equality between Nprotons and Nneutrons is altered again for the same reason; although you have the asymmetry term you get larger coulomb interactions between the protons...
So in general it's a competition between two extremes and the most stable one is the one that relatively stands in the middle.
 
  • #4
Orodruin said:
It is a carbon isotope. Isotope does not mean it cannot have the same number of protons and neutrons. Just that nuclei with a given number of protons are isotopes of the same element.

What isotopes are stable varies with the number of protons. For relatively light elements, you need roughly the same number of protons and neutrons. For heavier elements there is usually more neutrons in the stable isotopes.

Without you actually referencing a text, it is impossible to say whether it is the statement or your interpretation of it that id incorrect.
It's from a CGP book, quote ''In general, the greater the number of neutrons compared with the number of protons, the more unstable the nucleus.''
 
  • #5
It's from a CGP book, quote ''In general, the greater the number of neutrons compared with the number of protons, the more unstable the nucleus.''
In general there are more isotopes with extra neutrons than isotopes with too few neutrons. The above statement is too vague to be useful.
 
  • #6
Tangeton said:
It's from a CGP book, quote ''In general, the greater the number of neutrons compared with the number of protons, the more unstable the nucleus.''
That statement is wrong.
It applies to nuclei with more neutrons than in the stable region, but limited like that it becomes trivial - the more you go away from the stable region, the shorter the lifetime gets. This is true for both directions.
 
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  • #7
If you are neutron-rich, adding more neutrons makes things worse. If you are neutron-poor, adding more neutrons makes things better.
 
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  • #8
I guess there would be some elements for which there is no stable isotope, just some isotopes with a longer half life than most.
 
  • #9
Everything above lead (82 protons) does not have any stable isotope, and lead-208 is the heaviest isotope where no decay has been observed.
Zirconium-92 with 40 protons and 52 neutrons is the heaviest nucleus where no decay is possible at all. Everthing above that should be instable, but sometimes with lifetimes so long that the decay has not been observed yet.
 
  • #10
Is it possible to accurately predict the half life of an isotope in theory?
I know that there are good reasons to propose that Isotope X should be more stable than isotope Y.
However, can we reasonably well make a prediction for an isotope's half life, or is that only able to be ascertained by experiment.
 
  • #11
Depends on what you call "accurately". For alpha and beta decays, it is often not hard to get it right within an order of magnitude, and better calculations can be better than a factor of 2. Making more accurate predictions is really challenging - nuclei are very complex. As soon as you can observe the decay, experimental values are more precise than theoretical expectations.
 
  • #12
mfb said:
That statement is wrong.
It applies to nuclei with more neutrons than in the stable region, but limited like that it becomes trivial - the more you go away from the stable region, the shorter the lifetime gets. This is true for both directions.
Perhaps it was referring to fission products? There it is generally true that the fission products have an overabundance of neutrons.
 

1. How do neutrons affect the stability of an isotope?

The number of neutrons in an isotope can greatly affect its stability. Isotopes with a higher number of neutrons will typically be less stable, while those with fewer neutrons will be more stable.

2. Why do isotopes with more neutrons tend to be less stable?

This is because the strong nuclear force, which holds the nucleus together, must overcome the repulsive force between the positively charged protons. As the number of neutrons increases, so does the repulsive force, making it more difficult for the strong nuclear force to keep the nucleus stable.

3. Are there any exceptions to the rule that more neutrons make an isotope less stable?

Yes, there are some cases where adding more neutrons can actually make an isotope more stable. This is because the additional neutrons may help to balance out the repulsive force between protons, making the nucleus more stable overall.

4. How do scientists determine the stability of an isotope?

Scientists use various models and equations to calculate the stability of an isotope based on its ratio of protons to neutrons. They also look at the half-life of the isotope, as a shorter half-life typically indicates a less stable isotope.

5. Can the stability of an isotope be changed by altering the number of neutrons?

Yes, the stability of an isotope can be changed by adding or removing neutrons. This process is known as nuclear transmutation and is often used in nuclear reactors to create more stable isotopes or to produce radioactive isotopes for medical and industrial purposes.

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