What is the number of atoms in a mole of any pure substance referred to as?

The number of atoms in a mole of any pure substance is referred to as Avogadro's number, which is approximately (6.022 \times 10^{23}) entities (atoms, molecules, ions, etc.) per mole. This fundamental constant is crucial in chemistry as it provides a bridge between the microscopic world of individual atoms and the macroscopic scale where substances are measured in grams or moles.

Avogadro's number is derived from the work of Amedeo Avogadro, who first postulated that equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules. This principle holds true for all types of substances, making it a universal constant in stoichiometry.

The other terms listed do not pertain to the quantity of atoms in a mole. The atomic number refers to the number of protons in the nucleus of an atom and is unique to each element. The mass number is the total number of protons and neutrons in an atom’s nucleus. The isotopic number relates to specific isotopes of an element, which have different numbers of neutrons. Thus, Avogadro's number stands out as the correct choice in this context, defining the quantity of particles in a mole