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Is a single radon-daughter atom in air a solid?

The Wikipedia article on radon says:

Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as dust particles in the air. If such contaminated dust is inhaled, these particles can also cause lung cancer.

The statement made me wonder about the right terminology. Is a single radon-daughter atom in air, like 218Po, a solid or a gas? I would think it is a gas because it resembles a vapor atom, or a sublimated atom from a solid. And maybe it is a 'potential' solid?

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When the Wikipedia article says that radon daughters are "solids", the authors actually mean, "If you get a bunch of radon daughter atoms together, then they would form a solid." The state of matter is a property of a large number of atoms, so a single atom in isolation doesn't strictly have a well-defined state.

That said, states of matter are primarily a function of the interactions between atoms. Atoms that weakly interact* with themselves and their environment are likely to be gases, while atoms that strongly interact* with other atoms are likely to be liquids or solids. So Wikipedia appears to be using the state of matter as a shorthand for the strength of interactions. Essentially, radon daughters, unlike radon (which is a noble gas), stick to each other and to the walls, which is the same property that makes large collections of radon daughters solids.

*"weakly" and "strongly" don't refer to the fundamental weak and strong nuclear interactions here, of course, but to the general idea of having a small or large coupling constant in whatever interaction you're examining.

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  • $\begingroup$ "Atoms that weakly interact* with themselves " I think you mean "with each other". $\endgroup$
    – Acccumulation
    Jun 19, 2019 at 20:28
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A single atom is not a solid, a liquid, or a gas. These three terms refer to how large numbers of atoms or molecules behave, and they have no meaning for a single atom.

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After the decay of Rn-222 in air (i.e. not necessarily in case of Rn-222 that is trapped in minerals), the freshly generated Po-218 is neither a solid nor a gas in the usual sense. It is predominately present as positively charged ions. These ions are not stable and react quickly with atmospheric gases and form small clusters. Furthermore, the Po-218 ions can become neutralised by interactions with gases or other airborne substances, or Po-218 can attach to particulate aerosols.

The ionized form of radon progeny can be demonstrated in a simple experiment using a charged sheet of plastic that collects the ionized radon progeny from the air. The activity collected on the plastic can then be measured using a suitable radiation detector.

Finally, at normal atmospheric pressure and temperature, elemental polonium and typical polonium compounds are solids. After decay of Rn-222, however, there is not enough Po-218 present to form a discrete solid phase. The chemical concentrations of Po-218 are so low that the Po-218 atoms cannot find other Po-218 atoms. Thus, the Po-218 sticks to foreign material (usually first to particulate aerosols or other available surfaces).

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It starts as an ion, and it's soon going to become a molecule. I don't know anything about the chemistry of Polonium ions or atomic Polonium (!) but when it decomposes to Lead I think it will end up as PbO2 pretty fast. I'd hazard a wild guess that PoO2 can exist: it's a sort of ozone analogue.

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  • $\begingroup$ PoO2 definitely exists, like SO2 is. The +4 oxidation state is the most stable for polonium, while +6 is less stable (like +4 for lead, +5 for bismuth, etc.) $\endgroup$
    – trolley813
    Jun 20, 2019 at 19:33
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Every solid or liquid also evaporates a little bit, so there's nothing unusual to have a gas form of a material that prefers a solid form at that temperature. In fact, there's an equilibrium vapour pressure that a material wants above its surface, it's just extremely low for solids, and it's extremely rare for an atom/molecule to spontaneously leave a solid piece without some help. But if it was created in gas form (from radon), it's gas until it sticks somewhere, just like water vapour in air can exist even though it's below boiling point, until it rains or condenses on a mirror.

To conclude - the statement is written awkwardly, it would be better to state "radon's daughter elements form solids at room temperature, and will stick to surfaces..."

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