> From a totally naive point of view it seems like whether matter is a solid, liquid or gas largely has to do with how those atoms behave as a group.

That's correct, phases of matter are properties of large groups of atoms.

Your example of a uranium atom suspended in other matter might not be the best to make this point, because mixtures of different substances can make things more complicated. For example, pure sugar at room temperature is a solid. Add it to a cup of water, and it's a liquid solution. Add much, much more sugar and you get phase separation with some solid and some sirup-y liquid.