A representative particle is the smallest unit in a specific substance that naturally exists, which is typically an atom. For example, the representative particle in carbon is a carbon atom, likewise with iron and helium.
Although representative particles are usually atoms, that is not always the case. Some elements, according to CK-12 Chemistry, are never found in nature as single atoms. These elements, which include hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine, are found as diatomic molecules, which means that two atoms are bonded together. In these cases, the representative particles are the diatomic molecules. Another exception are molecules that exist in nature, such as water and carbon dioxide. Their representative particles are the water molecule and the carbon dioxide molecule, respectively. The representative particle for an ionic compound is its formula; in the case of sodium chloride, the representative particle is NaCl.
Because representative particles are so small, the concept of a "mole" was introduced to make sense of them. The mole equals 6.02 times 10 to the 23rd power representative particles. A mole of water molecules has 6.02 times 10 to the 23rd power water molecules in it. A mole of elephants includes 6.02 times 10 to the 23rd power of elephants.