Final answer:
The arrangement of particles in NaCl is most ordered in its solid state (NaCl(s)), with ions arranged in a face-centered cubic structure.
Step-by-step explanation:
The arrangement of particles is most ordered in a solid-state, such as NaCl(s), which has a highly organized lattice structure. In sodium chloride, NaCl, the sodium ions and chloride ions are arranged in a face-centered cubic (FCC) structure. The lattice is composed of alternating Na+ and Cl− ions, with each ion surrounded by six ions of the opposite charge. This highly orderly arrangement maximizes the interaction of opposite charges and is described by a 1:1 stoichiometry, with the unit cell of the FCC lattice containing four sodium ions and four chloride ions.