technitium

[emcelhannon]

Why is technitium radioactive?

Why is the atomic mass of potassium greater than argon, nickle greater than cobalt?

Edited December 10, 2009 by emcelhannon

[toastywombel]

Technitium has no stable isotope, because of this it will decay over time. During this decay it will release subatomic particles in the form of radiation.

 

The atomic mass of Potassium is not greater than Argon, Nickel's atomic mass is also not greater than that of Cobalt:

Potassium AM: 39.098

Argon AM: 39.948

Nickel AM: 58.70

Cobalt: 58.9332

 

I think you may have had it backwards and the reason Nickel's atomic mass is less than Cobalt's, and Potassium's is less than Argon's is because the atomic weight is determined by measuring the atomic mass of each isotope. Then weighing each of the isotopes masses by that isotopes abundance in nature. Therefore, the drop in atomic weight is due to the distribution of isotopes for those elements being different

 

This is from Wiki-Answers:

 

"Cobalt just has one isotope with any significant abundance: cobalt-59. Nickel on the other hand has several isotopes with significant abundances: nickel-58 (68%), nickel-60 (26%), nickel-61 (1%), nickel-62 (4%), nickel-64 (1%). Because the largest contributor to the atomic weight of nickel is the nickel-58 isotope, which is lighter than cobalt-59, the overall atomic weight comes out light despite nickel having an extra proton."

[emcelhannon]

Thanks Toasty,

I completely reversed the question on mass. Thanks for the correction and explanation. On technium however, I understand that it has no stable isotope, but I don't understand why. It's easy to accept the instability of heavy atoms, though I don't fully understand why. I would assume it involves a weight to energy ratio?

I might as well ask why 83 and up are unstable, but I'm especially currious about what makes 43 so different from the 20 before and after.

[swansont]

I might as well ask why 83 and up are unstable, but I'm especially currious about what makes 43 so different from the 20 before and after.

 

That's an easier question. You have to look at the modes of decay: there are beta-type decays, where you have a neutron changing into a proton or vice-versa, along with the creation of a lepton and anti-lepton, one of which is charged, to allow for satisfying various conservation laws. These occur when energetically permitted, because there is either a neutron or proton with significantly more energy than it would have if the decay occurred. because of the energy is pairing of spins, nuclei with an even number of neutrons or protons tend to have a slightly lower energy than if they have an odd number, so even numbers are energetically favored.

 

There are shells, just as in electron orbitals, which represent a lower energy, and give rise to "magic numbers" of particles.

 

There is also alpha decay, where an alpha particle tunnels out of the nucleus. These are prominent in heavy nuclei, because a reduction in the electrostatic repulsion results in a lower energy of the system.

 

So roughly speaking this tells you that there should be a preferred neutron/proton ratio; this will change as Z increases, because the energy of neutrons vs protons changes with the increasing electrostatic repulsion. And that even-even nuclei are more likely to be stable (there are no stable odd-odd nuclei heavier than Nitrogen) Heavy nuclei are more likely to be unstable, and we observe that that any nucleus above Z=82 (a magic number of protons) is indeed unstable.

 

So why is Z=43 unstable? It's got an odd number of protons, so that's a start, but I don't think that you could have made an a priori prediction of "no stable isotopes" without some very advanced modeling, or know if our modeling is up to that particular challenge.

[emcelhannon]

Thank you.

"Advanced modeling" must mean I wouldn't understand. Even though that's cheating, I'm accepting it, and moving on.

OK, now my curiosity has drifted to the pairing of spins and how it affects the energy of the nucleus. I don't know anything about it. How does that work?

[swansont]

Thank you.

"Advanced modeling" must mean I wouldn't understand. Even though that's cheating, I'm accepting it, and moving on.

OK, now my curiosity has drifted to the pairing of spins and how it affects the energy of the nucleus. I don't know anything about it. How does that work?

 

Yeah, "advanced modeling" means "works in the field of nuclear structure," meaning I have no clue.

 

The basic concept is that proton or neutron pairs in one state have to have anti-aligned spins because of the Pauli exclusion principle.

 

There are a whole bunch of ways different angular momentum states can interact; all of these particles have magnetic moments due to spin and orbital states. From a very gross standpoint you can probably see that aligned magnets have a different interaction energy than anti-aligned magnets. In atoms and nuclei, these interactions are small compared to the electric and nuclear forces, so they account for changes in energy that are small on the scale of the total interaction.