lonelind t1_jc9wjbg wrote

Gas, liquid, solid — it’s mostly about how close molecules/atoms are to each other (if you abstract enough from interatom interactions). Gaseous state means that everything is so far away and moves so quickly in random, it doesn’t feel any kind of interaction from outside. For it to happen you either need to reduce the distance (increase pressure) or to take away some energy from particles (cool down), so they could “talk” to each other. It’s called condensation and unless it happens to radon, you can easily think of atoms of lead separated from it as single atoms, it’s unlikely that they will condense together by themselves which will allow you to think of it as liquid/solid. Otherwise, you can think of it as a really expanded gas diffused into radon. Depends on what you’re trying to achieve and which processes to describe.