Radon-222 is an alpha emitter. During the alpha decay nucleus emits a 4-Helium particle (2 protons and 2 neutrons) and, as a result, the product of the decay is a different element, with atomic number 2 less. The chemical properties, including the state, is determined by the latter, thus the change. The alpha decay is sometimes also referred as a alpha-transformation due to the change of the chemical element in the process.

Dear Dr Miernik

You are indeed right. I only add this comment that other decays such as isobaric decays (beta minus, beta plus and EC) also change the atomic number and produce new elements.

I meant SOME other decays such as isobaric decays (beta minus, beta plus and EC) also change the atomic number and produce new elements. Isomeric transitions (gamma and IC) do not change the atomic number.

Like mentionned by M. Miernik, Radon-222 progenies are simply different elements. They are the results of the alpha emitting processes which decrease the atomic number and can then have a different state.

Radon gas mostly goes in and out of our lungs. But solid radon progenies can attach to dust or smoke and settle in the lungs.

Radon, regardless of the number of neutrons (hence, regardless of mass) is atomic number 86 and this is what gives it the chemical properties, making it a noble gas.

For something to be actually a gas, it needs to be more than a single molecule, so we consider a larger number of molecules (in the case of noble gases it's monoatomic molecules, so, unbound single atoms) which interact among themselves as a gas at the temperature of our choice.

From this perspective, you can see many species of atoms forming molecules with themselves as a system are a gas, liquid or solid depending on the temperature. Some never become real solids, but if you imagine a temperature of 0.0 K where nothing moves, you might be tempted to consider that system a sort of solid where a molecule always has the same neighbors in a kind of fixed positions.

Nevertheless, when you have a decay like SF, alpha or cluster emission, you eject daughter nuclei from the parent nuclei, one by one. In the case of alpha decay of Rn222 (which comes from the alpha decay of Ra226 - a "solid") the products are Po218, then Pb214 accompanied by He4 at each iteration.

So, a large number of monoatomic Radon molecules forming a (noble) gas ejects pairs of Helium nuclei that quickly pick up electrons forming monoatomic Helium molecules becoming (noble) gas and Polonium nuclei that pick up electrons forming Polonium atoms (NOT MOLECULES!!!) with a Pauling electronegativity of 2.0.

If at least 8 Po atoms do not get chemically bound with something else (like O, Br, Cl, etc. to form oxides or other stuff, ) and these atoms meet each other, then they form a cubic crystal at room temperature.

In conclusion, if you have a system of pure Rn monoatomic molecules (chemically inert) at room temperature forming a gas, then some Rn decays (over time) into pairs of He monoatomic molecules forming another gas (also noble gas, hence chemically inert) and Po atoms which bind into a cubic crystal, solid at room temperature. Since you only have inert Rn and inert He, the Po atoms can only bind to other Po atoms.

As a remark, the number of gas molecules in this system is constant in the first iteration, but after this, as Po alpha decays into Pb more He is produced.

If the initial system is not pure Rn gas, then the final chemical results may vary a lot. The resulting nuclei/atoms are always the same, but the molecules they form with whatever they encounter are not the same. This is always the real case, even if you do start with a pure Rn gas, because the decay chain of Rn produces several isotopes of some elements, thus opening the possibility of different chemical compounds (different sorts of molecules from stable or less stable Po, Pb, Bi, As, Tl or even Ti in the case of 220Rn) in different proportions depending on the probabilities and decay rates.

I hope this clarifies a bit the idea of a resulting gas, liquid or solid in ideal and in imaginary conditions, but also in the real world scenario.

The simple answer is as follwos. You see the physical propety is alsobased on the electronic configuration that in turn depends on number of protons. When alpha or beta decay occurs in any nculeus, its proton number changes and hence the electronic configuration is re-arranged accodingly. Naturaly the physical property also changes

Decay product depend on the chemical stste

Elements are defined as solid, liquid, or gas based on their state at standard temperature and pressure. All radon progeny are classed as solids at standard temperature and pressure. Nevertheless, any single atom or molecule suspended in air is in the gaseous state. All solids have a vapor pressure, however small, so all materials have a probability to be in a gaseous state. This probability is negligible for most solids, except those that are known to sublime near standard temperature and pressure. Polonium, a progeny of radon, has non-negligible vapor pressure at room temperature.

All radon progeny that are suspended in air or another gas as a single atom are in the gaseous state. The atom becomes a liquid or solid when it encounters a surface. Air has a very large surface area comprising molecular clusters and dust. The attachment time for radon progeny depends upon this surface area. The mean time to attachment is generally in the sub-micro-second range depending on the definition used for attachment (different investigators use different definitions and some ignore attachment to very small particles). (A complication in the definition of attachment is that the newly formed element has a positive electrical charge of about +10 and can become the center of a cluster.)

A very simple explanation: Radon is the last noble gas in the periodic table. Because noble gas electron structures have low affinity for reacting with other atoms or molecules, they exist in gaseous form. When radon decays it is no longer a noble gas, because it's outer shell is no longer fully occupied. That is why the decay products become solid.

I agree with the idea that radon is a noble gas and thus it forms few compounds, I have to stress that it forms few compounds as it has a full outershell of electrons. Unless you get something like silver ions which form a lewis acid/base complex with it it has no chemical bonds.

Thus it is in the form of single atoms, as they only have weak attractive forces to other things (such as london forces) it is very mobile and able to diffuse through many solids. One should note that it does not diffuse well through epoxy resins, polyethene or most paints.

After the radon undergoes a alpha decay it will then form Po which is much more able to form compounds.

I think that the fact that Ra-226 decays to a noble gas which then forms radioactive nuclides which bind to dust / smoke particles makes Ra-226 the real monster of a radionuclide that it is. I think that Ra-226 in many ways is much more dangerous Bq for Bq than Pu-239 or Am-241 becuase of this reason.

Radioactivity is a nuclear phenomena. It is number of protons in the nucleus which decides the 'State' of that element. An element can naturally exist in Solid, Liquid or Gas state. So, if by any means the number of protons in the nucleus varies ( be it by Alpha or Beta emission or by EC), the state of the element can also vary. Solid elements can change into Gas/Liquid or Gaseous ones into Solid/Liquid by radioactive transformation.