For a substance to occupy one half of an open container with the atoms being able to slide past each other, the most likely phase of the substance would be liquid.
Atoms of gases would not occupy just one-half of an open container because atoms of gases will diffuse and spread out from an open container.
Atoms of solids are fixed about a particular position and will most likely not be able to slide past each other in an open container.
Only the atoms of liquid take the shapes of their containers and do not have the capacity to diffuse out ordinarily. They are also able to flow despite the closeness of the atoms and would occasionally slide past each other.
