If you have a single atom in a box, what state of matter would it be in? If the walls of the box were made of a completely inert material, what state of matter would the atom be in? Or would it not have one? I realized this is a duplicate of several other questions.
 A: "States of matter," like other quantities in statistical mechanics, are only defined for collections of multiple atoms (or molecules or ions or other such constituents), and only really have sensible meaning once you have large numbers of atoms (or molecules, etc.). This is because a state of matter is characterized by its constituents' positions relative to each other and interactions with each other. 
For example, in a solid, constituents are closely spaced relative to each other, do not move much relative to each other, and interact via transmission of forces through a fixed lattice. In a liquid, constituents are still closely spaced, but move relative to each other, and interact via contact interactions transmitted through an amorphous collection of adjacent bodies. In a gas, constituents are spaced further apart, and interact little, mainly through collisions. In a plasma, constituents are still spaced further apart, but now interact via collisions and through longer-range electromagnetic interactions.
A single atom has no neighbors, and so its spacing from its neighbors and interactions with neighbors cannot be defined. As such, it has no definable state of matter.
