[quarks0010lss]

Why is technetium unstable when all the elements around it are stable?  Thanks

[Mitch]

No isotope of technetium is stable with respect to beta decay.

[shelanachium]

Look up a table of the known nuclei in a chemistry or physics databook. You would expect that if there were a stable isotope of technetium it would be around 97, 99 or 101. Now look up the stable isotopes of its neighbours 42 (molybdenum) and 44 (ruthenium). These being even-Z elements each have many stable isotopes, and these include 97, 99 and 100. So all the isotopes of Tc that might be expected to be stable decay either to Mo (by EC or beta+) or Ru (by beta-).

The same happens with Promethium (Z=61). All 'expected' stable isotopes of Pm decay to Nd or Sm in a similar manner.

It nearly happens with Indium (Z=49); its neighbour tin (Z=50) having no fewer than 10 stable nuclides. In fact we know that the commonest isotope of In is radioactive, and decays to Sn-115, but with an amazingly long half-life (10^15 yrs). The other, rarer isotope, In-113, is just stable.

[quarks0010lss]

Perhaps my question is not clear.  What causes Tc to be radioactive while the surrounding elements Cr, Mn, Fe, Ru, Os, Re, W, and Mo all have stable isotopes.  What is it about this light element that causes its instability.  With its small nucleus how is the strong force overcome.

Thanks again.

[Mitch]

Its not a strong force issue, the process of its decay is by beta-decay thus through the weak force. Promethium is also unstable, but the spontaneous process is slower and it just takes a very long time for it to decay away.

Your questions is hard to answer without writing a lot of information.
http://en.wikipedia.org/wiki/Technetium#Stability_of_technetium_isotopes

[dysprosia]

Its not a strong force issue, the process of its decay is by beta-decay thus through the weak force. Promethium is also unstable, but the spontaneous process is slower and it just takes a very long time for it to decay away.

Your questions is hard to answer without writing a lot of information.
http://en.wikipedia.org/wiki/Technetium#Stability_of_technetium_isotopes

This very question piqued my interest and resulted in my registering to be a member of Chemical Forums. For forty years I have thought about it. So far I have not seen a satisfactory answer. Nuclear models such as shell theory and liquid drop theory do not predict this as far as I know. My mathematical knowledge is limited to basic calculus so bear with me if you can show that the above theories do indeed predict the radioactivity of Tc and Pm.
As far as I am aware the pat answer that the surrounding elements have all the possible stable nuclear arrangements locked up so to speak is akin to the following. Ten books are placed on a table and one is held in mid air. Everything in the system is subject to a gravitational field. When the eleventh book is no longer held it accelerates downward. The experimenters, not knowing about the existence of gravity, pat answer is that all the other books on the table left no room for the eleventh book and thus it fell.

[renge ishyo]

I very much like shelanachium's answer, but I will add a bit on a point he briefly touched on if it helps any.

There is a general trend that atoms with even atomic numbers have more stable isotopes than those with odd atomic numbers. If you flip through a book that lists all of the stable isotopes for each atom (a good one to do this with is Elmsley's "The Elements"), you will notice that the atoms 1~88 with odd atomic numbers have 1 or at most maybe 2 stable isotopes listed for them. For example, Manganese has an atomic number of 25 and only 1 out of the 4 isotopes listed is stable, and Iodine has an atomic number of 53 and only 1 out of the 5 isotopes listed is stable. In contrast the atoms with even atomic numbers tend to have two or more than two stable isotopes (and sometimes many more). For instance, Magnesium with atom number 12 has 3 stable isotopes out of the 4 isotopes listed, and Selenium with an atomic number of 34 has 6 stable isotopes out of the 7 isotopes listed.

The reason for this likely has to do with the nuclear packing of the protons (which are equal in number to the atomic number). If you have an odd number of protons you can't arrange them in a symmetrical pattern very easily. To get a feel for why this is try making a symmetrical object like a large square by stacking small square blocks closely together. You will find that you can only make "smooth" squares by using an even number of blocks; if you use an odd number, there will always be a "zit" hanging out somewhere in your arrangement.

For the nucleus, having an odd "man" out (or proton in this case) means that the process of introducing neutrons to try and "fix it" is harder than it would be if you just had an even number of protons to start out with. The evidence for this is that these odd numbered atoms tend to be much more sensitive to the number of neutrons present than the even numbered atoms are.  For example, for Cobalt if you have exactly 32 neutrons present, then the isotope is stable. On the other hand, Cobalt is unstable if you have 33, 31, 30, or 29 neutrons present. For even numbered atoms the number of neutrons present is usually much less of an issue as can be seen with Neodymium which is stable if it has 82,83.85,86,88, or 90 neutrons while only configurations with 84 and 87 neutrons are unstable.

So what does this all mean for technetium? It means that it is just statistically unlucky for the most part. Unlike Cobalt which can cover up the problem with that *perfect* number of neutrons so that it has *at least one* stable isotope, none of the numbers of neutrons available to technetium can be arranged in such a way to make technetium isotopes stable. If we "outlawed" 32 neutrons in the nucleus, then Cobalt too would be in the same category of "stability" as technetium because all it's other neutron possibilities are unstable. Some odd numbered atoms such as Bromine are "above the average" as they ended up with 2 isotopes that are stable, many like cobalt had only one stable isotope, and some like technetium ended up "below the average" and had none. I suppose that's just the way things go in this world.

Hope this helped.

[dysprosia]

I'm sorry if my questions and speculations are tedious. If so let me know or if you do know where someone is willing to discuss these things at length I would appreciate you telling me.
The following is what I have hypothesized over the years:
Considering the odd man or "zit". Is it then supposed that the proton which is having difficulties getting along with the rest of the nuclei is one that freely moves about in the nucleus? Are we then saying that the strong force still holds the nucleons but periodically allows one of them to stray far enough from the main nuclear body to allow the weak force to predominate? The amount of time spent in the outer limits of the nucleus would then determine the decay rate. The meta stable Tc nuclei apparently are higher energy symmetrical nuclear arrangements somewhat akin to electrons in excited states. Furthermore is this evidence that there is a tendency in all nuclei to form alpha particle groups and arrange these into one or more symmetrical shapes?

[renge ishyo]

Well dysprosia, part of the problem is nobody seems to agree all too well about what sort of structures or arrangements exist or don't exist in the nucleus and it makes it hard to discuss it without the science police coming in to say "you can't suggest that". I have a hunch that the "odd man out" in the nucleus is something akin to having an extra electron left over in an orbital (which normally makes an electron easier to remove from atoms than the situations where every electron is paired up with another in it's orbital), but that's all it is...a hunch. Nothing solid backing it. I remember reading about some theories of nuclear energy levels and such in books such as Linus Pauling's "general chemistry," but these views are otherwise apparently non-mainstream.

I am personally hoping that the LHC will yield enough new data for scientists that a more detailed description of the nucleus can eventually be formed (although I guess first I should hope that the thing actually works sometime soon).

[dysprosia]

Thank you, Renge. I have also read speculations and theories about nuclear structure and most of it is more than fifty years old. There are also some unusual ideas posted on the internet. I was hoping that with the various methods used to produce atoms up to element 120 that there might be additional information available.