Here's why:

* Ionic Compounds: Sodium ions are formed when a sodium atom loses an electron, resulting in a positively charged ion. Sodium ions typically exist in ionic compounds, where they are attracted to negatively charged ions (like chloride ions in table salt, NaCl).

* Representative Particle: The term "representative particle" refers to the smallest unit of a substance that retains the chemical properties of that substance. For ionic compounds, this is the individual ion.

So, although sodium ions might exist in a crystal lattice with other ions, the individual Na+ ion is the representative particle of a sodium ion.