A mole contains an exact number of specified particles.

The counting unit packages exactly 6.02214076 × 10²³ specified elementary entities.

Those entities may be atoms, molecules, ions or other explicitly identified particles.

The mole measures amount of substance, not a fixed mass or volume.

A dozen tells you how many objects you have without telling you their individual size. The mole performs a comparable job on a scale suited to atoms and molecules. Chemists rarely count those particles one at a time; instead, they weigh samples, measure concentrations, or analyze electrical charge. The mole links those laboratory observations with the particle ratios in chemical equations. Its enormous numerical size reflects how tiny individual atoms are compared with the quantities handled in ordinary experiments.

The earlier SI definition tied the mole to the number of atoms in twelve grams of carbon-12. That relationship made carbon a reference point connecting atomic mass measurements to macroscopic material. On 20 May 2019, a revised definition made the particle count itself exact. This change separated the definition of amount of substance from the definition of mass. It preserved established chemical practice closely enough that laboratory recipes did not suddenly require different quantities of their ingredients.

Specifying the entity prevents a common misunderstanding. One mole of oxygen atoms is not the same sample as one mole of oxygen molecules. Ordinary oxygen gas contains O₂ molecules, each with two oxygen atoms, so one mole of those molecules contains two moles of oxygen atoms. Likewise, a mole of sodium chloride formula units describes a ratio of sodium and chloride ions in a crystal, not necessarily a collection of separate floating NaCl molecules. The description of what is counted belongs with the number.

Molar mass supplies the bridge to a balance. Water has a molar mass of approximately eighteen grams per mole, while carbon dioxide is approximately forty-four grams per mole. Equal amounts of substance therefore need not have equal masses. They also need not occupy equal volumes: liquids, solids, and gases pack particles differently. Even for gases, equal-volume comparisons require specified temperature and pressure and an appropriate approximation to ideal behavior. The mole itself makes no promise about the physical dimensions of a sample.

Consider the balanced equation in which two hydrogen molecules react with one oxygen molecule to form two water molecules. The same coefficients describe two moles of hydrogen reacting with one mole of oxygen to produce two moles of water, if the reaction proceeds as written. They do not mean two grams, one gram, and two grams. This distinction is why amount of substance is useful in reaction calculations, medication preparation, and solution chemistry. An exact counting definition still leaves experimental uncertainty in sample purity, mass measurements, and the molar masses used to convert between scales.

Concentration adds another distinction: one mole per liter describes amount divided by solution volume, not a new particle-count definition. Preparing such a solution requires attention to the final total volume, rather than simply adding a mole of solute to a liter of solvent.

The mole is a counting unit with a specified kind of entity. Saying that a sample contains a mole is incomplete unless the entities are identified: atoms, molecules, ions or another stated group. The count can be fixed even when the mass of those entities differs.