By Joy Shepperson, updated March 24 2022
A representative particle is the smallest unit of a substance that can be divided without changing its chemical composition. All matter can be described in terms of three fundamental representative particles: atoms, molecules, and formula units.
Atoms are the tiniest discrete units that retain a substance’s identity. A pure substance composed of a single atom type is an element, such as hydrogen, oxygen, or iron.
A molecule consists of two or more atoms covalently bonded together. Molecules serve as the representative particles for all covalent (molecular) compounds, including the diatomic elements H₂, O₂, N₂, etc.
Ionic solids are electrically neutral aggregates of cations and anions. The smallest neutral unit that reflects the true stoichiometry of an ionic compound is called its formula unit. For example, the formula unit of sodium chloride is NaCl, representing the 1:1 ratio of Na⁺ to Cl⁻.
Certain elements naturally exist as diatomic molecules (two identical atoms bonded together) under standard conditions. These species are not part of a larger compound but are distinct molecules themselves.
