mali73 t1_jcab1m0 wrote
Reply to comment by stefek132 in Radon is a monatomic gas, but its decay products are solids. After a decay, what happens to the individual atoms of the daughter elements? Do they stay suspended in the atmosphere or slowly rain out? by foodtower
"Even [diatomic radical reactions] must fulfil strict geometrical (sic) conditions in order to pair lone electrons" I don't agree with. There is genuinely only 1 geometric paramater, i.e. the distance between the two atoms, unless you are implying Born-Oppenheimer doesn't apply or relativistic effects are necessary (which, they are, but for which your bouncing ball model doesn't account). Even the angle between their velocities does not matter following from Newtonian relativity.
I also believe your "energy threshold" (Eyering transition state Gibbs free energy?) is being imagined as far too high. Ground state monoatomic oxygen and lead must overcome only Pauli repulsion in the formation of the transition state, which will be on the same order of magnitude as Coulombic benefit entering the transition state, so I posit ca. 10% of bond enthalpy as the maximum barrier? Which for even the strongest diatomic leaves us with 90 kJ/mol, which for an ordinary reaction proceeds unmonitorably quickly at room temperature, for a more reasonable guess of 20 kJ/mol we need an argon matrix to observe these using IR spectroscopy. Reaction with triplet and heaven forbid singlet dioxygen will likely have larger barriers. Do I think these barriers will be large enough to stop reaction with a radon daughter lead? No not at all. Even under your assumptions.
Saying it will only occur when it forms larger clusters is frankly laughable. The ionisation energy goes up, the Gibbs free enegy of the transition state goes up, and frankly the chance of interacting with oxygen goes down, not up when forming a cluster. The total reactive lead surface area goes down rapidly with cluster size as core atoms become inaccessible, and on forming a cluster the volume goes down from overlapping atomic radii. On small clusters the cluster radius is a similar fraction of the mean free path as the atomic radius of a lead atom is of the mean free path, so forming a Pb(0) tetramer roughly quarters your reaction time. Lead metal sheets oxidises slowly, and can readily be chemically forced back to metal, however fine lead powder can ignite simply by throwing it through air.
Your assumption that lead clusters are forming in preference to reaction with oxygen is only true if your amount of parent radon is decaying much faster than it is diffusing or you have far more radon than oxygen. This is not true, as isolable radon (222Rn) has a half life of almost 4 days. Under any reasonable conditions this is much slower than diffusion and all radon will be well mixed with the surrounding air.
I would be frankly shocked to see any metallic lead deposition at all. I tried to read about experimental evidence for this however daughter isotope measurement is only performed by radiation measurments rather than any chemical means so the identity of the material is never stated (comprehensive article by Yamamoto et al. in J. Environ. Radioactivity).
Bbrhuft t1_jcajnfa wrote
FYI. have a radium dial compass sealed inside an air tight jar, safely stored in an unoccupied room. It's highly radioactive, back then Zinc sulfide phosphor wasn't particularly sensitive so they compensated by adding extra radium.
Anyway, the interior of the jar gets coated with radon daughter plate out:
This is the decay I measured, due to Bismuth-214 and Lead-214 decay.
Anyways, the contamination stubbornly adhers to the glass. I tried rubbing it off with tissues, dampened with water and alcohol. I estimate I can remove about 25% of the contamination, by measuring the radioactivity on the tissue, most remains stuck to the glass.
Radon Plate Out occurs because the decay products (218Po, 214Pb and 214Bi) are electrically charged, they are attracted to dust and surfaces that are slightly charged.
mergelong t1_jcapyht wrote
I find that pretty interesting, but I imagine that especially the upper atmosphere, with high levels of ionizing radiation and radicals floating around, doesn't resemble inside of a jar, not to mention the distance the daughter nuclides must travel before deposition is vastly increased for atmospheric radon decay products.
Bbrhuft t1_jcat9ql wrote
Radon Daughters stick to dust at ground level and that dust is carried into the higher atmosphere by rising air currents, they can rain out when there's heavy rain, thunderstorms particularly, a phenomena called Radon Washout.
It was discovered by accident in the 1960s. A nuclear worker walked though puddles in a car park on the way to work, and he set off the alarms as he arrived, since that's backwards they were intrigued, and they discovered that atmospheric dust is coated with radon daughters which can get concentrated in electrically charged thunderstorms, and rain out as Radon Washout.
Radon Washout can sometimes be intensely radioactive, and there's a paper that estimated that a few percent of skin cancers might be linked to Radon Washout, beta radiation from Lead-214 and Bismuth-214 decay is able to penetrate the outer layers of the skin and deposit a radiation dose to living skin cells, a risk increased for people who work outdoors. This might be speculative, nevertheless, it illustrates just how radioactive rain can be sometimes be when weather conditions are just right.
I measured it myself a few times. Got readings up to 2 microsieverts per hour, nothing spectacular.
Styro, B.I. and Stelingis, K.I., 1978. On the value of flow of long-lived radon-222 decay products into atmosphere with the dust of natural and anthropogenic origin. In Chemical and radioactive pollution of the atmosphere and hydrosphere. V. 4.
Edit: Also, >90% of indoor radon daughters are bound to dust, very little is unbound, free floating.
drsoftware t1_jcb2kj7 wrote
Another writer responded with mean free path and velocity for particles. Given that dust, that is other larger masses than individual oxygen atoms, will the electrically charge particle be much more likely to first bound with oxygen and then with dust?
[deleted] t1_jcb3omc wrote
[removed]
Bbrhuft t1_jcfday0 wrote
One important consideration is the fact that Lead-214 and Bismuth-214 are electrically charged, singly ionized ions. The other factor that needs to be taken into account is the fact that these reactions occur within an ionization trail that contains hydroxyl radicals and NO2 (the Alpha particles are ejected at 7-8% the speed of light, and the daughter atoms recoil at 10 to 13 million mph). And the final factor that make all this more complex that single atoms floating in nitrogen and oxygen is the fact that air isn't pure, but is often contaminated with volatile organic compounds, particularly indoor air, and SO2.
As a result Lead and bismuth oxide isn't formed. They form clusters of 5 to 8 water molecules and often a few molecules of VOCs. They were measured experimentally, and are 1.2 - 2 nanometers in diameter.
Po-218 is a little different, it can form larger clusters about 15 nm that additionally contain SO2 (if air contains a few ppm of SO2, common in urban and inner city environments), and water molecules. It can also form much smaller clusters of PoOx, 0.5 - 1 nm.
I think I've summarize what I read correctly.
Ref.:
Castleman Jr, A.W., 1991. Consideration of the chemistry of radon progeny. Environmental science & technology, 25(4), pp.730-735.
Hopke, P., 1996. The initial atmospheric behavior of radon decay products. Journal of Radioanalytical and Nuclear Chemistry, 203(2), pp.353-375.
mali73 t1_jch9lo4 wrote
Yes, you're right with compelling literature. I should've looked into the actual conditions the daughter joins are formed under but I typed it at 1am. I get rilled up if I see poor reasoning and tend to go after it without consideration of context and probably should've stopped after the first paragraph.
Viewing a single comment thread. View all comments