What does Avogadro's number represent?

Avogadro's number, which is approximately (6.022 \times 10^{23}), represents the number of particles—such as atoms, molecules, or ions—in one mole of a substance. This fundamental concept in chemistry allows us to connect the macroscopic properties of materials to their microscopic composition.

When using the mole as a unit, Avogadro's number provides a bridge between the mass of a substance and the actual quantity of entities present. For example, when dealing with one mole of oxygen gas (O(_2)), Avogadro's number tells us that there are (6.022 \times 10^{23}) molecules of O(_2) in that mole. This understanding is crucial for performing stoichiometric calculations, as it allows chemists to quantify reactants and products in chemical reactions based on the number of particles involved.

While other options relate to important concepts in chemistry, they do not accurately define Avogadro's number. For instance, the mass of one mole of a substance involves molar mass, while the volume of gas at STP specifically pertains to the ideal gas law, and the number of atoms in a molecule relates to molecular structure, not to the mole concept