Why Elements 119+ May Not Exist

[pierce15]

Hello,

Is there a quantum mechanical reason why elements 119+ might not exist? i.e. would the crazy orbitals associated with these elements violate the laws of physics in any way?

 

[Simon Bridge]

They would certainly be very unstable.
It is not known if there is a specific law of physics restricting the upper size of a nucleus - the range of the strong nuclear force would seem to indicate there should be some limit - exploring this is what finding new elements is all about. I don't think anyone expects 119 to be the cutoff point: properties for elements 165-167 have been predicted using existing knowledge of physics for eg.

 

[Drakkith]

The repulsive force from all those protons simply dominates over the short range nuclear/strong force, making it more favorable for superheavy elements to split into lighter ones. Generally, the heavier the element, the quicker it will decay on average. Some combinations of nucleons are supposed to be "magic numbers", which would make the more stable relative to isotopes with non-magic numbers of nucleons, but even then they are still very short lived.

 

[pierce15]

How are we able to find the theoretical half life of these atoms? Also, why are there certain stable isotopes?

 

[Simon Bridge]

We use quantum mechanical models to work out what sort of properties to expect. We compare these with the measured properties to see if we (a) have the particle in question, and (b) need to modify the model.

The stability-valleys for the "magic numbers" are related to particular resonances in these models.
What education level do you need this at? These are topics typically covered at senior undergrad, and above at college.

You could try:
http://people.chem.duke.edu/~jds/cruise_chem/nuclear/stability.html

 

[pierce15]

I have a bunch of non-relativistic quantum mechanics under my belt. That website didn't really go in detail on the more complicated, interesting stuff. Is chromodynamics necessary to study these large atoms?

 

[e.bar.goum]

Hello,

Is there a quantum mechanical reason why elements 119+ might not exist? i.e. would the crazy orbitals associated with these elements violate the laws of physics in any way?

Not really! You've got to distinguish between "atom" and "nucleus" here - there is reason to suspect you will begin to not be able to make neutral atoms from relativistic quantum mechanics, but careful analysis suggests not.

But, what we actually care about is whether we can produce nuclei with Z>119. And there is no law that says we won't be able to - I work in a research group that spends a lot of time examining the reactions that you can use to produce Z>119 (but I work on the other end of the nuclear chart). In fact, it is only thanks to quantum mechanical models that superheavy elements can exist - you need, at the very least, the shell model.

Up until now, we've used reactions of 48Ca (a doubly magic nucleus) on 248Cm, 249Cf or 249Bk to produce Z =116 - 118 respectively. However, this won't work for Z = 119. The most popular candidate for 119 is 50Ti on 249Bk. But it's not a simple matter to decide which two nuclei to smash together!

The trouble is, is that we're very good at producing compound nuclei that are a little proton rich, but the models tell us that any island of stability will be on the neutron rich side of the chart - so we need to add more neutrons into the system. Unfortunately, it's not as simple as just chucking in more neutrons - nuclear structure considerations need to be taken into account, which change the candidate reactions.

The other difficulty is that the odds of making these nuclei are incredibly tiny. You need the best part of a year of beam-time to produce maybe 6 of your candidate nuclei, which is why there are only 3 places in the world that can produce new elements (LBNL, Dubna, GSI). Further, isotopes like 48Ca, Bk, and Cf are incredibly hard to produce, so there is only one or two places you can get the ingredients! (Oak Ridge, USA and Dimitrovgrad, Russia).

Another(!) difficulty is that you don't discover these elements by directly detecting the new nucleus. What you do is wait for them to decay via alpha-emission to a known nucleus, which will then decay via alpha-emission to another known nucleus, and so on and so on, so you've got to pick out the signals of these decays in amongst all of the other signals that you're getting on the detectors from the beam - one obvious choice would be to pulse the beam, and only look for the decays when the beam is "off", which is what happens, but these superheavies are actually relatively long lived - on the order of microseconds - so that becomes a big pain. Digital electronics have been a great help here, as are sophisticated magnetic filters.

Phew. And as to whether you can predict the half-lives? Hah! The trouble is that nuclear models are pretty awful at predicting this kind of thing away from stability. We don't actually know for sure where we expect to find the next shell closure (magic nucleus). If you ask 4 different theorists, you'll get 4 different answers. Possibly more if you put them in a room together and argue.

 

[Simon Bridge]

An atom is an atom even if it is not neutral.

 

[e.bar.goum]

Ah, but to be on the periodic table, you need to be concerned about the electron configuration of the neutral atom, which is why chemists care whether or not you can, in principle, form a neutral atom.

Nuclear physicists? Electrons? Feh, who cares.

 

[pierce15]

Thanks for the info!