When is a particle classified as an atom a molecule or formula unit?

A particle is classified as an atom when it consists of a single unit of an element, such as a hydrogen or oxygen atom. It is classified as a molecule when two or more atoms are chemically bonded together, like in H₂O or O₂. A formula unit refers to the simplest ratio of ions in ionic compounds, such as NaCl, representing the compound's overall composition without defining the molecular structure. Each classification reflects the particle's composition and bonding characteristics.