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Radon is an extremely unstable noble gas. Due to this, putting it in a gas discharge lamp for a prolonged period of time is extremely hard, as pure radon is needed to give a true color, but radon decays, contaminating the lamp. But suppose we were to find a way to not let radon decay, concentrate it in a gas discharge lamp, and turn the lamp on. What color would the radon gas glow?

Sources I find are contradictory. images-of-elements.com says

Radon is said to glow red in discharge tubes

and the image that it gives

is only an illustration, not radon itself.

Wikipedia says that it "glows green or red in discharge tubes", but it doesn't cite any sources there.
The spectral lines of radon are listed in physics.nist.gov/PhysRefData/Handbook/Tables. According to this source, radon gives probably somewhat reddish due to the strong lines at 705 and 745 nm.

reddit: what_color_does_radon_glow_when_electricity_is says that

Radon glows violet/purple when discharged. The reason why is simply we can infer it off of the patterns of the periodic table. You see, radon is at the very bottom to the right, in the noble gases column. Noble gases are already stable (for the most part) and do not want to react with other particles; thus requiring a lot of energy to react. As radon is the largest natural noble gas element, we humans can mess with it a little bit more than its other noble gas brethren. Now since we know it requires a lot of energy to react, we know it is related to light with an insanely short wavelength (shorter wavelength=higher energy). If we look at the light spectrum we can see the shortest wavelength and highest energy wavelength is purple/violet/indigo. Thus making radon that color when discharged.

And based on the emission spectrum from chemistry.bd.psu.edu, it appears that radon would fluoresce with a deep bluish-violet color.

Same question for oganesson. Although Wikipedia says it should be a solid due to relativistic forces, what color would oganesson glow in a gas discharge lamp?

orthocresol
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Max0815
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    The reddit comment seems just bizarre, given the prevalence of neon lights (particularly in Vegas, although most lighting seems to be LED there now). – Jon Custer Jun 20 '19 at 23:39
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    Interesting question! While I don't have an answer, there are stars that emit lines from technetium... for a relatively short time. ;-) https://en.wikipedia.org/wiki/R_Geminorum . See also the Capitol Steps song, Everything Glows :https://www.deseretnews.com/article/338958/CAPITOL-STEPS-BRING-MUSICAL-MOCKERY-TO-SL.html – DrMoishe Pippik Jun 21 '19 at 02:03
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    As there would be glowing, melting or vaporization due very intense alpha bombardment, such a question is rather funny. – Poutnik Dec 23 '21 at 08:46
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    @Buttonwood Rn VIS emission line intensities are provided in the NIST link in the question. – Poutnik Dec 23 '21 at 09:48
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    According to Rutherford, a discharge of Radon (radium emanation) had a bluish color when he measured it. He also notes that the color of the discharge changed to 'pale rose' when the radon was condensed using liquid air. – Paul Dec 23 '21 at 11:45
  • @Poutnik Paschen's law tells us that a glow discharge will work best in a pretty low pressure gas. Would a thin glass tube with say 10 Torr of radon inside be melting or vaporizing, or visibly glowing under self-ionization or incandescence? Maybe so, that would be worth finding out! – uhoh Dec 23 '21 at 13:27
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    @uhoh It is quite clear, but even low pressure with half-life 3.6 days of 222Rn means enormous radioactivity, ionization and thermal effects, not speaking about oganesson. – Poutnik Dec 23 '21 at 14:44
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    @uhoh I did some quick calculation and unless I made error, decaying of 1 mol of radon gives power about 1.2 MW. If we take 1 L at 10 torr, it gives about 700 W. It look like radon plasma even without external voltage. – Poutnik Dec 23 '21 at 14:53
  • @Poutnik Yikes! I see what you mean. Subconsciously I was probably thinking of Rn as an everyday household substance and most houses don't glow from it, but yes those partial pressures are a dozen orders of magnitude or more lower than our glow discharge tube. Thanks! – uhoh Dec 23 '21 at 21:15

1 Answers1

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The Royal Society of Chemistry has a Twitter account (here) and they have shared the spectrum of radium emanation as it was known in the beginning. It is a beautiful picture showing the comparison of helium with the newly discovered gas.

In photographic spectroscopy, darkness of the lines indicated intensity. One can see the intense lines at 416 and 418 nm and around 460 nm. So the emission must be appearing violet blue in a discharge tube. Like most elements, the ground state transitions, hence the strongest, are in the UV region.

The reasoning at reddit and other unauthentic sources of information are not reliable and close to gibberish.

Of course, NIST is the most reliable source today, however the line 745 nm is on the very edge of visible deep deep red. It is very hard to see, just like potassium, one cannot easily see the deep red lines, hence the flame appears violet/lilac to us.

enter image description here

AChem
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  • *beautiful!* – uhoh Dec 27 '21 at 05:33
  • The argument here is that the behavior of Ra and K generalize to other elements, in particular Rn. However, since you selected radium, not radon, as an example to illustrate your point, and the names are potentially confusing in being so similar, not to mention that the figure compares Ra to He, a noble gas like Rn, you might want to clarify the point. Or is the point to potentially confuse? – Buck Thorn Feb 06 '22 at 08:41
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    Please read the label carefully, it is "Radium emanation" not radium, which is an old name for Radon. – AChem Feb 06 '22 at 14:32
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    Thank you for that clarification, I learned something new. I should also have read your answer more carefully, "...as radon was known in the beginning..." – Buck Thorn Feb 06 '22 at 21:40
  • @BuckThorn, I like chem history, that is why it was a familiar term. Emanation was common name for a radioactive gas from actinide elements too. Those turned out to be isotopes of Rn. – AChem Feb 07 '22 at 03:33