Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.


Recommended Posts

I have been wondering about the stability of atomic nuclei and what makes them stable/unstable.

As an example, if I have an atomic nucleus containing 7 protons and 7 neutrons, that is nitrogen-14 and is stable. If I change one proton for one neutron I have carbon-14 which is unstable. So what is it that makes one stable and the other unstable? The C-14 is slightly heavier, but that is by less than 0.01% which I don't see would have an enormous impact. The other obvious difference is that the N-14 has one more positive charge, which would appear to be a source of increased repulsion, so that would seem to imply at face-value it should be less stable rather than more so. Other than that, I am beginning to flounder in my search for differences.

So does anyone have an answer as to why specific nuclei are stable or unstable?


Share this post

Link to post
Share on other sites

Straight from Wikipedia:

Physical magic numbers and odd and even proton and neutron count[edit]

Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain magic numbers of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. As in the case of tin, a magic number for Z, the atomic number, tends to increase the number of stable isotopes for the element.

Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many even-even nuclides into another even-even nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy. This makes for a larger number of stable even-even nuclides, which account for 152 of the 253 total. Even-even nuclides number as many as three isobar (nuclide)s for some mass numbers, and up to seven isotopes for some atomic (proton) numbers.

Conversely, of the 253 known stable nuclides, only five have both an odd number of protons and odd number of neutrons: hydrogen-2 (deuterium), lithium-6, boron-10, nitrogen-14, and tantalum-180m. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40, vanadium-50, lanthanum-138, and lutetium-176. Odd-odd primordial nuclides are rare because most odd-odd nuclei are highly unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.[2]

Yet another effect of the instability of an odd number of either type of nucleons is that odd-numbered elements tend to have fewer stable isotopes. Of the 26 monoisotopic elements (those with only a single stable isotope), all but one have an odd atomic number, and all but one has an even number of neutrons — the single exception to both rules being beryllium.

The end of the stable elements in the periodic table occurs after lead, largely due to the fact that nuclei with 128 neutrons are extraordinarily unstable and almost immediately shed alpha particles. This also contributes to the very short half-lives of astatine, radon, and francium relative to heavier elements. This may also be seen to a much lesser extent with 84 neutrons, which exhibits as a certain number of isotopes in the lanthanide series which exhibit alpha decay.


Share this post

Link to post
Share on other sites

Thanks for this - it is going to take a bit of digestion.

But the first thing that occurs is that, albeit that N-14 is one of the five stable odd-odd isotopes, C-14 is even-even, which should prevent decay to N-14, whilst C-13 (6p+7n) is stable, so there must surely be more to the story ...

Share this post

Link to post
Share on other sites

The link provided finally led me here, which goes a long way to providing an explanation as to why C-14 (even-even) is not stable, but N-14 (odd-odd) and C-13 (even-odd) are. Thanks.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.